issue with update statement

[luvmy2Js]

Could someone please help me out? I have an update statement that I am trying to do and it keeps giving me this error:

Msg 156, Level 15, State 1, Line 3
Incorrect syntax near the keyword 'CONVERT'.???

I have no clue what I am doing wrong..

Here is my code:

UPDATE ystdy_PA0000
SET
CONVERT(nvarchar, DecryptByKey([Action Type])) = 'Z1'
WHERE SAPID IN (910959, 520651, 520806, 910192, 910395, 910909, 520405, 525169, 910767, 521402, 522105, 522438, 521401, 520404, 521626)
and CONVERT(nvarchar, DecryptByKey([Action Type])) = 'ZI'


These are encrypted fields..

Thanks a million!!

 

[PWise]

UPDATE ystdy_PA0000
SET fieldname = 'Z1' WHERE SAPID IN (910959, 520651, 520806, 910192, 910395, 910909, 520405, 525169, 910767, 521402, 522105, 522438, 521401, 520404, 521626) and CONVERT(nvarchar, DecryptByKey([Action Type])) = 'ZI'

 

[luvmy2Js]

When I try that I get this error:

Implicit conversion from data type varchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.