VARBINARY conversion error when passing a NULL

[amachanic]

Describe the bug

When passing a NULL to a VARBINARY column for insert, an error occurs rather than the correct behavor.

To reproduce

import mssql_python

conn = mssql_python.connect([conn string], autocommit=True)
cur = conn.cursor()
cur.execute('create table bytestest (b varbinary(50));')

cur.execute('insert into bytestest values (?)', None)

Exception message:

ProgrammingError: Driver Error: Syntax error or access violation; DDBC Error: [Microsoft][SQL Server]Implicit conversion from data type varchar to varbinary is not allowed. Use the CONVERT function to run this query.

Expected behavior

The None should be inserted as a NULL and I should see no error about VARCHAR.

[github-actions]

Hi @amachanic, thank you for opening this issue!

Our team will review it shortly. We aim to triage all new issues within 24-48 hours and get back to you.

If you have additional information to share, please feel free to update the issue.

Thank you for your patience!

[sumitmsft]

Hi @amachanic

We've confirmed this is a valid bug. The root cause is in how the driver handles NULL parameter type mapping. When you pass None as a parameter value, the driver currently always maps it to SQL_VARCHAR at the Python layer, regardless of the target column type. Since SQL Server does not allow implicit conversion from VARCHAR to VARBINARY, this causes the error you're seeing.

Thanks,
Sumit