item_timefunc.cc:

  manual adding the fix for bug#15828 which did not automerge

sql/item_timefunc.cc

@@ -3023,9 +3023,9 @@ void Item_func_str_to_date::fix_length_and_dec()
   cached_field_type= MYSQL_TYPE_STRING;
   max_length= MAX_DATETIME_FULL_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
   cached_timestamp_type= MYSQL_TIMESTAMP_NONE;
-  if ((const_item= args[1]->const_item()))
+  format= args[1]->val_str(&format_str);
+  if (!args[1]->null_value && (const_item= args[1]->const_item()))
   {
-    format= args[1]->val_str(&format_str);
     cached_format_type= get_date_time_result_type(format->ptr(),
                                                   format->length());
     switch (cached_format_type) {