Range Selection Error

I am having some trouble with a variable range selection within a regression. I keep getting an "application-defined or object-defined error." I've isolated each statement to find that the code that is causing the regression not to work is below (the error for that line of code states that the Select method of Range class failed):

Sheets("Index").Range("$A$1:$D$" & regressionWindow)

This is confusing because the following code works in a different module:

Sheets("Index").Range("$A$1:$D$" & indexMonthsCount)

This is the full regression code for the one that is not working:

Application.Run "ATPVBAEN.XLA!Regress", Sheets("Transpose") _
        .Range("$A$ & counter3:$A$" & regressionWindow), Sheets("Index") _
        .Range("$A$ & counter3:$D$" & regressionWindow), _
        False, True, , "fund", False, False, False, False, , False

(regressionWindow = counter3 + 36)


Thank you for any help you can offer on this issue