diff --git a/src/sqlalchemy_cratedb/dialect.py b/src/sqlalchemy_cratedb/dialect.py
index 79ea3a3..1e7aefd 100644
--- a/src/sqlalchemy_cratedb/dialect.py
+++ b/src/sqlalchemy_cratedb/dialect.py
@@ -114,10 +114,10 @@ class DateTime(sqltypes.DateTime):
 
     def bind_processor(self, dialect):
         def process(value):
-            if value is not None:
-                assert isinstance(value, datetime)
+            if isinstance(value, (datetime, date)):
                 return value.strftime('%Y-%m-%dT%H:%M:%S.%fZ')
-            return value
+            else:
+                return value
         return process
 
     def result_processor(self, dialect, coltype):