SumIf WorksheetFunction referencing another workbook

Dear all,

I'm trying to use the SumIf worksheetfunction in VBA, looking in another workbook, but I apparently use the wrong reference. It works without any problem in Excel. I receive the following pop up message : "method range of object worksheet failed"

Here is my code :

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I understand that to be proper, I should use workbooks(File).worksheets("Budget 2013").range("Q5:Q52"), but I'm afraid that my code will be too long, because I'd like to use many conditions (with SumIfs)...
I already tried many attempts without any success, would you have any idea about my mistake?

Many thanks in advance for your lights.

Best regards,

Eric