Expression and Constraint

Hi,
I have a Execute SQL Task that outputs a resultset. I would like to check the output variable (which in this case is of the type object) using the "Expression and Constraints" Evaluation option of the Precedence Constraint if it is empty before running the next task.
Tried "!ISNULL(@ReturnVal)" which returned , "The data type of variable not supported in expression" error.
Is there a way to evaluate object variable in an expression??
Appreciate any help.
Thanks