Incorrect syntax near DecryptByPassPhrase issue

can anyone help to find the issue in the query. I am using sql server 2005.

This query is working fine:

SELECT DecryptByPassPhrase('SHA1', ColName) FROM Table

but when i use:

SELECT isnull(convert(varchar(200) DecryptByPassPhrase('SHA1', ColName)), '')  
FROM Table

it gives error:

Incorrect syntax near 'DecryptByPassPhrase'. Severity 15

i also tried where clause WHERE colName IS NOT NULL expecting it is because of null values but same error.