Type mismatch

This is to calculate a bonus for our technicians based on how many hours they bill. Some technicians get an additional bonus for hours worked on refrigeration equipment and some don't. 
I'm getting a type mismatch from my IF statement, so I added a tonumber but am still getting the same error. Any ideas?
I've tested the parts of my formula separately and they work as far as I can tell, it's just the IF statement that's giving me grief.

SumValues(GetRecords($myquery,[_DBID_QSALESHISTORY]),23)+[Hours Adjustment]+
If([Refrigeration Rate]="",
(tonumber(SumValues(GetRecords($aquery,[_DBID_QSALESHISTORY]),23)+[Refrigeration Hours Adjustment])))

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Anne Martin
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