Cryptic Sql Execution Error

I am trying to create a Union view with the following query:

SELECT TxType, AccNo, Stream
FROM dbo.vw7_Existing_Member_History
UNION
SELECT TxType, AccNo, Stream
FROM dbo.vw8_Deleted_Member_History

Here TxType is a short int, AccNo a nvarchar and stream is also nvarchar.

Stream containd values sch as "MCA", "MBA", "B.Ed." etc....

When I try to execute this query I get the following error:
Error Message: Conversion failed when converting the nvarchar value 'B.Ed.' to data type int.


Removing Stream from both clauses of the query leads to uccessful execution.
As I said, Stream is nvarchar, and nowhere am I trying to convert it into an int!!!!

What does this error means then???