It means somewhere within that procedure something's trying to convert a sql_variant to varchar.
Can you post the procedure?
Why are you using SQL_variants anywhere?
How are you constructing the parameter list in your front end code? (C#?)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability