diff --git a/src/meds/schema.py b/src/meds/schema.py
index 3df6b44..dfa97c7 100644
--- a/src/meds/schema.py
+++ b/src/meds/schema.py
@@ -21,7 +21,7 @@
 birth_code = "SNOMED/184099003"
 death_code = "SNOMED/419620001"
 
-def events_schema(patient_id_type, custom_per_event_properties=pa.null()):
+def events_schema(patient_id_type, custom_per_event_properties=[]):
     assert patient_id_type == pa.int64() or patient_id_type == pa.string()
     
     return pa.schema(