diff --git a/api-impl/src/main/kotlin/dev/bpmcrafters/processengineapi/impl/task/VariablesExt.kt b/api-impl/src/main/kotlin/dev/bpmcrafters/processengineapi/impl/task/VariablesExt.kt
index d333f8f..d6f09bf 100644
--- a/api-impl/src/main/kotlin/dev/bpmcrafters/processengineapi/impl/task/VariablesExt.kt
+++ b/api-impl/src/main/kotlin/dev/bpmcrafters/processengineapi/impl/task/VariablesExt.kt
@@ -7,7 +7,7 @@ package dev.bpmcrafters.processengineapi.impl.task
  *
  * @since 0.0.1
  */
-fun Map<String, Any>.filterBySubscription(subscription: TaskSubscriptionHandle): Map<String, Any> =
+fun Map<String, Any?>.filterBySubscription(subscription: TaskSubscriptionHandle): Map<String, Any?> =
   if (subscription.payloadDescription != null) {
     if (subscription.payloadDescription.isEmpty()) {
       mapOf()
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/PayloadSupplier.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/PayloadSupplier.kt
index d354f73..2590ea1 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/PayloadSupplier.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/PayloadSupplier.kt
@@ -3,7 +3,8 @@ package dev.bpmcrafters.processengineapi
 import java.util.function.Supplier
 
 /**
- * Supplier for the payload.
+ * Supplier for the payload of commands to the engine 
+ * Used to for instance define process-variables when starting processes.
  * @since 0.0.1
  */
-fun interface PayloadSupplier : Supplier<Map<String, Any>>
+fun interface PayloadSupplier : Supplier<Map<String, Any?>>
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/CorrelateMessageCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/CorrelateMessageCmd.kt
index bd58cef..1b7e9bd 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/CorrelateMessageCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/CorrelateMessageCmd.kt
@@ -30,7 +30,7 @@ data class CorrelateMessageCmd(
    * @param payload payload to use.
    * @param correlation correlation to use.
    */
-  constructor(messageName: String, payload: Map<String, Any>, correlation: Correlation, restrictions: Map<String, String>) :
+  constructor(messageName: String, payload: Map<String, Any?>, correlation: Correlation, restrictions: Map<String, String>) :
     this(
       messageName = messageName,
       payloadSupplier = PayloadSupplier { payload },
@@ -44,7 +44,7 @@ data class CorrelateMessageCmd(
    * @param payload payload to use.
    * @param correlation correlation to use.
    */
-  constructor(messageName: String, payload: Map<String, Any>, correlation: Correlation) :
+  constructor(messageName: String, payload: Map<String, Any?>, correlation: Correlation) :
     this(
       messageName = messageName,
       payloadSupplier = PayloadSupplier { payload },
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/SendSignalCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/SendSignalCmd.kt
index 5602740..ea2ecec 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/SendSignalCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/correlation/SendSignalCmd.kt
@@ -26,7 +26,7 @@ data class SendSignalCmd(
    * @param restrictions restrictions.
    * @param payload payload to use.
    */
-  constructor(signalName: String, restrictions: Map<String, String>, payload: Map<String, Any>) : this(signalName, PayloadSupplier { payload }, restrictions )
+  constructor(signalName: String, restrictions: Map<String, String>, payload: Map<String, Any?>) : this(signalName, PayloadSupplier { payload }, restrictions )
   /**
    * Constructs a signal command by signal name, restrictions and no payload.
    * @param signalName signal name.
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/decision/DecisionByRefEvaluationCommand.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/decision/DecisionByRefEvaluationCommand.kt
index d004b43..f9154a9 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/decision/DecisionByRefEvaluationCommand.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/decision/DecisionByRefEvaluationCommand.kt
@@ -28,7 +28,7 @@ data class DecisionByRefEvaluationCommand(
    * @param payload payload to use.
    * @param restrictions restrictions for the message.
    */
-  constructor(decisionRef: String, payload: Map<String, Any>, restrictions: Map<String, String>) : this(
+  constructor(decisionRef: String, payload: Map<String, Any?>, restrictions: Map<String, String>) : this(
     decisionRef = decisionRef,
     payloadSupplier = PayloadSupplier { payload },
     restrictionSupplier = { restrictions }
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByDefinitionCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByDefinitionCmd.kt
index e738827..1b35384 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByDefinitionCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByDefinitionCmd.kt
@@ -26,7 +26,7 @@ data class StartProcessByDefinitionCmd(
    * @param payload payload to use.
    * @param restrictions restrictions for the message.
    */
-  constructor(definitionKey: String, payload: Map<String, Any>, restrictions: Map<String, String>) : this(
+  constructor(definitionKey: String, payload: Map<String, Any?>, restrictions: Map<String, String>) : this(
     definitionKey = definitionKey,
     payloadSupplier = PayloadSupplier { payload },
     restrictions = restrictions
@@ -36,7 +36,7 @@ data class StartProcessByDefinitionCmd(
    * @param definitionKey process definition key.
    * @param payload payload to use.
    */
-  constructor(definitionKey: String, payload: Map<String, Any>) : this(
+  constructor(definitionKey: String, payload: Map<String, Any?>) : this(
     definitionKey = definitionKey,
     payloadSupplier = PayloadSupplier { payload } ,
     restrictions = mapOf()
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByMessageCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByMessageCmd.kt
index 36780c2..ae7b63f 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByMessageCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/StartProcessByMessageCmd.kt
@@ -26,7 +26,7 @@ data class StartProcessByMessageCmd(
    * @param payload payload to use.
    * @param restrictions restrictions for the message.
    */
-  constructor(messageName: String, payload: Map<String, Any>, restrictions: Map<String, String>) : this(
+  constructor(messageName: String, payload: Map<String, Any?>, restrictions: Map<String, String>) : this(
     messageName = messageName,
     payloadSupplier = PayloadSupplier { payload },
     restrictions = restrictions
@@ -36,7 +36,7 @@ data class StartProcessByMessageCmd(
    * @param messageName message name.
    * @param payload payload to use.
    */
-  constructor(messageName: String, payload: Map<String, Any>) : this(
+  constructor(messageName: String, payload: Map<String, Any?>) : this(
     messageName = messageName,
     payloadSupplier = PayloadSupplier { payload },
     restrictions = mapOf()
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/ChangePayloadModifyTaskCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/ChangePayloadModifyTaskCmd.kt
index fb1c49c..412209f 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/ChangePayloadModifyTaskCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/ChangePayloadModifyTaskCmd.kt
@@ -32,7 +32,7 @@ abstract class ChangePayloadModifyTaskCmd(
          * @param taskId id of the task to modify.
          * @param payload payload to use.
          */
-        constructor(taskId: String, payload: Map<String, Any>) : this(
+        constructor(taskId: String, payload: Map<String, Any?>) : this(
             taskId = taskId,
             payloadSupplier = PayloadSupplier { payload },
         )
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskByErrorCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskByErrorCmd.kt
index 66ee906..2f2a93e 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskByErrorCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskByErrorCmd.kt
@@ -31,7 +31,7 @@ open class CompleteTaskByErrorCmd(
    * @param errorMessage Optional details.
    * @param payload payload to use.
    */
-  constructor(taskId: String, errorCode: String, errorMessage: String?, payload: Map<String, Any>) : this(
+  constructor(taskId: String, errorCode: String, errorMessage: String?, payload: Map<String, Any?>) : this(
     taskId = taskId,
     errorCode = errorCode,
     errorMessage = errorMessage,
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskCmd.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskCmd.kt
index b209462..c48268c 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskCmd.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/CompleteTaskCmd.kt
@@ -22,7 +22,7 @@ open class CompleteTaskCmd(
    * @param taskId id of the task to complete.
    * @param payload payload to use.
    */
-  constructor(taskId: String, payload: Map<String, Any>) : this(taskId, PayloadSupplier { payload } )
+  constructor(taskId: String, payload: Map<String, Any?>) : this(taskId, PayloadSupplier { payload } )
 
   /**
    * Creates the complete command for a given task id without payload.
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskHandler.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskHandler.kt
index f0603d5..99a50d1 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskHandler.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskHandler.kt
@@ -7,9 +7,9 @@ import java.util.function.BiConsumer
  * @since 0.0.1
  */
 @JvmDefaultWithoutCompatibility
-fun interface TaskHandler : BiConsumer<TaskInformation, Map<String, Any>> {
+fun interface TaskHandler : BiConsumer<TaskInformation, Map<String, Any?>> {
 
-  fun process(parameters: Pair<TaskInformation, Map<String, Any>>): Pair<TaskInformation, Map<String, Any>> {
+  fun process(parameters: Pair<TaskInformation, Map<String, Any?>>): Pair<TaskInformation, Map<String, Any?>> {
     accept(parameters.first, parameters.second)
     return parameters
   }
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskModification.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskModification.kt
index 155a456..1040cd4 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskModification.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/TaskModification.kt
@@ -40,7 +40,7 @@ class TaskModification(private val taskId: String) {
    * @param payload new payload.
    * @return builder instance.
    */
-  fun updatePayload(payload: Map<String, Any>) = this.apply {
+  fun updatePayload(payload: Map<String, Any?>) = this.apply {
     commands += UpdatePayloadTaskCmd(taskId, payload)
   }
 
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/AssignmentDetector.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/AssignmentDetector.kt
index 5b5c4b2..5699030 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/AssignmentDetector.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/AssignmentDetector.kt
@@ -13,5 +13,5 @@ interface AssignmentDetector {
    * @param newTaskInformation new task information and payload.
    * @return true, if assignment change is detected. Defaults to `false`.
    */
-  fun hasChangedAssignment(oldTaskInformation: Pair<TaskInformation, Map<String, Any>>, newTaskInformation: Pair<TaskInformation, Map<String, Any>>): Boolean = false
+  fun hasChangedAssignment(oldTaskInformation: Pair<TaskInformation, Map<String, Any?>>, newTaskInformation: Pair<TaskInformation, Map<String, Any?>>): Boolean = false
 }
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/CompositeTaskHandler.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/CompositeTaskHandler.kt
index 2eacae4..098cdbd 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/CompositeTaskHandler.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/CompositeTaskHandler.kt
@@ -28,7 +28,7 @@ class CompositeTaskHandler(
     return CompositeTaskHandler(handlers.plus(handler).toMutableList())
   }
 
-  override fun accept(taskInformation: TaskInformation, payload: Map<String, Any>) {
+  override fun accept(taskInformation: TaskInformation, payload: Map<String, Any?>) {
     handlers.fold( Pair(taskInformation, payload)) { params, handler -> handler.process(params) }
   }
 
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupport.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupport.kt
index b833ef4..2871704 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupport.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupport.kt
@@ -19,7 +19,7 @@ class UserTaskSupport(
 ) : TaskHandler, TaskTerminationHandler {
 
   private lateinit var subscription: TaskSubscription
-  private val payload: ConcurrentHashMap<String, Map<String, Any>> = ConcurrentHashMap()
+  private val payload: ConcurrentHashMap<String, Map<String, Any?>> = ConcurrentHashMap()
   private val information: ConcurrentHashMap<String, TaskInformation> = ConcurrentHashMap()
   private val compositeTaskHandler: CompositeTaskHandler = CompositeTaskHandler().withHandler(this)
   private val compositeTaskTerminationHandler: CompositeTaskTerminationHandler = CompositeTaskTerminationHandler().withHandler(this)
@@ -35,7 +35,7 @@ class UserTaskSupport(
    * @throws IllegalArgumentException if no task with given id is found.
    */
   @Throws(IllegalArgumentException::class)
-  fun getPayload(taskId: String): Map<String, Any> {
+  fun getPayload(taskId: String): Map<String, Any?> {
     return requireNotNull(payload[taskId]) { "Could not find any variables for task $taskId." }
   }
 
@@ -146,7 +146,7 @@ class UserTaskSupport(
    * @param taskInformation task information.
    * @param taskPayload payload of the task.
    */
-  fun onTaskDelivery(taskInformation: TaskInformation, taskPayload: Map<String, Any>) = compositeTaskHandler.process(taskInformation to taskPayload)
+  fun onTaskDelivery(taskInformation: TaskInformation, taskPayload: Map<String, Any?>) = compositeTaskHandler.process(taskInformation to taskPayload)
 
 
   /**
@@ -159,7 +159,7 @@ class UserTaskSupport(
   /**
    * Implement assignment detection.
    */
-  override fun process(parameters: Pair<TaskInformation, Map<String, Any>>): Pair<TaskInformation, Map<String, Any>> {
+  override fun process(parameters: Pair<TaskInformation, Map<String, Any?>>): Pair<TaskInformation, Map<String, Any?>> {
     val taskInformation = parameters.first
     val taskPayload = parameters.second
     // first modify the reason based on the assignment detector decision
@@ -186,7 +186,7 @@ class UserTaskSupport(
    * @param taskInformation task information.
    * @param taskPayload payload of the task.
    */
-  override fun accept(taskInformation: TaskInformation, taskPayload: Map<String, Any>) {
+  override fun accept(taskInformation: TaskInformation, taskPayload: Map<String, Any?>) {
     information[taskInformation.taskId] = taskInformation
     payload[taskInformation.taskId] = taskPayload
   }
diff --git a/api/src/test/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupportTest.kt b/api/src/test/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupportTest.kt
index d5d9ea1..fa3dfd3 100644
--- a/api/src/test/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupportTest.kt
+++ b/api/src/test/kotlin/dev/bpmcrafters/processengineapi/task/support/UserTaskSupportTest.kt
@@ -10,7 +10,7 @@ import java.util.*
 internal class UserTaskSupportTest {
 
   private val support = UserTaskSupport(assignmentDetector = object: AssignmentDetector {
-    override fun hasChangedAssignment(oldTaskInformation: Pair<TaskInformation, Map<String, Any>>, newTaskInformation: Pair<TaskInformation, Map<String, Any>>): Boolean {
+    override fun hasChangedAssignment(oldTaskInformation: Pair<TaskInformation, Map<String, Any?>>, newTaskInformation: Pair<TaskInformation, Map<String, Any?>>): Boolean {
       return oldTaskInformation.first.meta["custom-assignment-field"] != newTaskInformation.first.meta["custom-assignment-field"]
     }
   })