diff --git a/cdisc_rules_engine/check_operators/helpers.py b/cdisc_rules_engine/check_operators/helpers.py
index 4a9348b8e..10cf702e0 100644
--- a/cdisc_rules_engine/check_operators/helpers.py
+++ b/cdisc_rules_engine/check_operators/helpers.py
@@ -56,7 +56,7 @@ def default_value(self):
 
 
 def is_valid_date(date_string: str) -> bool:
-    if date_string is None:
+    if date_string is None or not isinstance(date_string, str):
         return False
     try:
         isoparse(date_string)