diff --git a/src/internal/proc.php b/src/internal/proc.php
index 4acf640..5cbadd9 100644
--- a/src/internal/proc.php
+++ b/src/internal/proc.php
@@ -53,10 +53,12 @@ function proc(string $process, ?LoopInterface $loop = null): PromiseInterface
 
   return new Promise(
     function (callable $resolve, callable $reject) use (&$data, $proc) {
-      $proc->stdout->on(
-        'error',
-        function (\Throwable $err) use ($reject) {
-          $reject($err);
+      $proc->stderr->on(
+        'data',
+        function (string $err) use ($reject) {
+          $reject(
+            new \Exception($err)
+          );
         }
       );
 
diff --git a/tests/Internal/InternalTest.php b/tests/Internal/InternalTest.php
index 54fef58..c560944 100644
--- a/tests/Internal/InternalTest.php
+++ b/tests/Internal/InternalTest.php
@@ -19,7 +19,7 @@ public function procProvider(): array
       // php commandline process
       [['php -r \'echo "foo";\''], 'foo'],
       // invalid input
-      [['kat --foo'], ''],
+      [['kat --foo'], "sh: 1: kat: not found\n"],
     ];
   }