diff --git a/redash/utils/__init__.py b/redash/utils/__init__.py
index eb99caba5d..b752bacb37 100644
--- a/redash/utils/__init__.py
+++ b/redash/utils/__init__.py
@@ -82,7 +82,7 @@ def default(self, o):
         elif isinstance(o, (datetime.timedelta, uuid.UUID)):
             result = str(o)
         # See "Date Time String Format" in the ECMA-262 specification.
-        if isinstance(o, datetime.datetime):
+        elif isinstance(o, datetime.datetime):
             result = o.isoformat()
             if o.microsecond:
                 result = result[:23] + result[26:]