What's wrong in this code?

[jeba]

Hi there!
I am getting the following error when I run the following SP.

Server: Msg 137, Level 15, State 1
[Microsoft][ODBC SQL Server Driver][SQL Server]Must declare the variable '@maxid'.

CREATE  Procedure spFindMax @tablename varchar(50), @pykey varchar(50)  As 
Declare @SqlStr varchar(1000) , @maxid int
set @SqlStr ='Select @maxid=ISNULL(max('+ @pykey  + '),0) From ' + @tablename
Exec (@SqlStr)
print @maxid

Please help me. Thanx in advance.