When performing a query with an aggregate function such as Count or Sum from MSAccess, an error is returned

When running the following query:
SELECT Count(tblContacts.contactID) AS CountOfcontactID, tblContacts.firstName
FROM tblContacts
GROUP BY tblContacts.firstName

The error returned is:
"The field is too small to accept the amount of data you attempted to add. Try inserting or pasting less data."

The error is is returned using Count, Sum, Avg, StDev and Var. But not when using Min, Max, First or Last.

This error is not generated if using the MySQL ODBC connector to connect to the same database.

connector log attached.