When using in selectable stored procedure data type Varchar(8191) I keep getting

Overflow occurred during data type conversion 

this is my domain 

CREATE DOMAIN DESCRIPTION

 AS Varchar(8191)

 COLLATE UNICODE_CI_AI; 

There is no conversion needed as the same domain is used in table field and stored procedure returns parameter. 

however when just doing select it works fine. but inside SP not. What might be the problem? 

FB 2.5.2