SQL Query Returns a Null Value When Checking for Not Numeric

I have a SQL query that I run against either Access, dBase or Excel. In that query I check for null fields and also, depending on user preferences, non numeric values in fields that should only contain numerics. The correct records are returned but fields with non-numeric entries have a null value returned instead of the actual contents. Is this normal or have I just messed up (which is very likely)? Here is a sample query:

SELECT * FROM [TableName] WHERE [Field1] IS NULL OR IsNumeric([Field2]) = False

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There could be several fields checked, some only for null and others for both null or non-numeric. All conditions are joined with "OR" as above.