Custom Functions Show #VALUE! After Macro Has Run

Recently I found this forum through a Google search along with the perfect solution to a problem I was having finding and replacing text based on a table of replacement values. Here is the thread:

Multiple substitute or replace text using a table

I incorporated the SuperSub function that steveorg developed as a result of that thread into my worksheet by creating a new module and pasting the following code into it:

Function SuperSub(OriginalText As String, rngOldText As Range)
Dim cel As Range
Dim strOldText As String, strNewText As String
' loop through list of old_text used in substitute
For Each cel In rngOldText.Cells
strOldText = cel.Value
strNewText = cel.Offset(0, 1).Value
OriginalText = Application.WorksheetFunction.Substitute(OriginalText, strOldText, strNewText)
Next cel
SuperSub = OriginalText
End Function

It works like a charm as a formula, but I also have a macro in the same workbook that, as soon as it selects the sheet with the formula, calls the SuperSub function and starts executing the code as a macro (even though there is no call to this routine in the macro). The macro ran just fine before incorporating the function into my worksheet.

The formula looks like this: =TRIM((supersub( UPPER(E2),rngSubst)))
If cell E2 contains the string "101 North Main Street, Apartment 5", it would return "101 N MAIN ST APT 5"

Here is the macro:

Sub Import()
'
Sheets("Shoebuy FTP").Select
Range("A2:R200").ClearContents 'This is where it jumps to Function SuperSub(OriginalText As String, rngOldText As Range)

The function runs for every occurrence of the formula in the worksheet, and when it is finished, all of the cells containing that formula show a value of "#VALUE!". I should mention that it does not change the formula at all, but I then have to highlight each cell that contains the formula, press <F2> to edit, then <ENTER> to get it to display the correct results again.

It is probably some stupid little thing that I didn't do when I created the function, but I can't figure it out. I have written numerous macros, but this is the first time I have ever created a custom function.