diff --git a/pandas/_libs/src/vendored/ujson/python/objToJSON.c b/pandas/_libs/src/vendored/ujson/python/objToJSON.c
index 5f35860c59cb7..4c89bd9540312 100644
--- a/pandas/_libs/src/vendored/ujson/python/objToJSON.c
+++ b/pandas/_libs/src/vendored/ujson/python/objToJSON.c
@@ -346,7 +346,8 @@ static char *PyDateTimeToIsoCallback(JSOBJ obj, JSONTypeContext *tc,
   }
 
   NPY_DATETIMEUNIT base = ((PyObjectEncoder *)tc->encoder)->datetimeUnit;
-  return PyDateTimeToIso(obj, base, len);
+  GET_TC(tc)->cStr = PyDateTimeToIso(obj, base, len);
+  return GET_TC(tc)->cStr;
 }
 
 static char *PyTimeToJSON(JSOBJ _obj, JSONTypeContext *tc, size_t *outLen) {