SQL Query with Excel VBA Returns General ODBC Error when Using Variables

I have set up an SQL query of an Excel Worksheet using VBA in Excel. The query works when I "hardcode" the search string, but when I use a variable, I get the following error:

Run Time Error: '1004'
General ODBC Error.

Here is the code with the variable xCaseNo.

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If I supply the CaseNo by typing it in a string enclosed in single quotes, the query works, as in:

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When I get the error message the oQt.Refresh line is highlighted in yellow.

I suspected the problem may be related to SQL requiring quotes around the search string, so I defined xCaseNo as

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, but that didn't work either.

Thanks.