stored procedures and interbase

I have created the following generator and stored procedure in interbase:

CREATE GENERATOR CONTX

CREATE PROCEDURE CONTAINERGENPRIME RETURNS (CONTAINERID INTEGER)
      AS
      BEGIN
      CONTAINERID = GEN_ID(CONTX , 1);
      END

but when I call execproc using tsqlstoredproc component I get an "operation
not supported" error.  What am I doing wrong?  This approach worked fine
using the bde stored proc object.  Thanks for your help.