MS Access Query - Help with <> in query

I have data that looks like this (simplified):

ID, Category
1,AB
2,CD
3,       <-- Blank or Null
4,EF
5,GH
6,AB
... 

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When I try to filter out "GH" the query is also ignoring the Blank or Null rows. Here is the query:

SELECT ID, CATAGORY
FROM TestTable
WHERE CATAGORY <> "GH";

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I have also tried this with the same results:

SELECT ID, CATAGORY
FROM TestTable
WHERE Not(CATAGORY = "GH");

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I am at a loss why the results include all Categories except for "GH" (expected) and blank or null rows. This is omitting several thousand records that should be part of the results.