Table Column Default

Hello all,

I have a table with a column:
ThruDate datetime not null

I have a function call set up as default for the col
([dbo].[fn_DateHigh_Get1]())


I have a stored procedure that inserts into the table - it expects @ThruDate as a parm. I default the parm to NULL -

When the sp executes its insert stmt i get error 515 (null cannot be inserted....).
When i just enter the row manually in enterprise manger (and just tab through the ThruDate col) all is well - ThruDate gets the default as calculated by function [dbo].[fn_DateHigh_Get1]()

Why does the sp fail - i.e. why isn't the default applied?

thanks