Item_param::save_in_field():

thd->command can be anything except COM_EXECUTE in no cases
except programming error.

sql/item.cc

@@ -580,8 +580,7 @@ int Item_param::save_in_field(Field *field, bool no_conversions)
 {
   THD *thd= current_thd;
 
-  if (thd->command == COM_PREPARE)
-    return -1;
+  DBUG_ASSERT(thd->command == COM_EXECUTE);
   
   if (null_value)
     return (int) set_field_to_null(field);