diff --git a/api/schemas.py b/api/schemas.py
index 314a0f7d2..fdea7f439 100644
--- a/api/schemas.py
+++ b/api/schemas.py
@@ -560,6 +560,8 @@ class _SessionSearchEventRaw(__MixedSearchFilter):
                 assert len(values["source"]) > 0 and isinstance(values["source"][0], int), \
                     f"source of type int if required for {PerformanceEventType.time_between_events}"
             else:
+                assert "source" in values, f"source is required for {values.get('type')}"
+                assert isinstance(values["source"], list), f"source of type list is required for {values.get('type')}"
                 for c in values["source"]:
                     assert isinstance(c, int), f"source value should be of type int for {values.get('type')}"
         elif values.get("type") == EventType.error and values.get("source") is None: