Type Mismatch with Workbooks.Open - excel

Question

I have a larger code set to help automate an auditing model. I am attempting to add a function to import a claims file. I have a "GetFiles" function I use regularly to import information but now I am receiving type mismatch error.
I have already tried different variable assignments and fully quantifying different items but it appears to err out regardless. Originally I was doing OpenFile = GetFiles (see commented line) and then my formula read as set aux = application.workbooks.open(OpenFile) but same error
This is a small section of the code so a number of the variables declared are not visible in this snippet.There is also some commented out lines regarding "ThisWorkbook" as the module will be moved from my personal working file into an actual template model.
Sub Audit_Template_Autofill()
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Dim column_id() As Variant, A1 As Range, Z As Range, c As Range
Dim column_count As Long, x As Long, input_option() As Variant, Loop_Input As Variant
Dim claims As Worksheet, audit As Worksheet, OpenFile As Variant, aux As Workbook
'run function to get claims file for import
'GetFiles
' Set claims = ThisWorkbook.Sheets("Claims Data")
' Set audit = ThisWorkbook.Sheets("Audit")
Set claims = ActiveWorkbook.Sheets("Claims Data")
Set audit = ActiveWorkbook.Sheets("Audit")
'OpenFile = GetFiles
Set aux = Application.Workbooks.Open(GetFiles)
ActiveWorkbook.ActiveSheet.Cells.Select.Copy
claims.Range("A1").Paste
Set A1 = claims.Range("A1")
Set Z = A1.End(xlToRight).Offset(0, 1)
column_count = claims.Range(A1, Z).Count
ReDim column_id(0 To column_count): ReDim input_option(0 To column_count)
For Each c In Range(A1, Z)
column_id(x) = c.Value
x = x + 1
Next c
x = 0
Do While x <= column_count
input_option(x) = x + 1 & "=" & column_id(x)
x = x + 1
Loop
x = 0
Do While x <= column_count
Loop_Input = Loop_Input & input_option(x) & vbCrLf & ""
x = x + 1
Loop
the error occurs on the set aux = Application.Workbooks.open(GetFiles)
Here is the Function GetFiles
Function GetFiles() As Variant
GetFiles = Application.GetOpenFilename(Title:="Select Files(s)", MultiSelect:=True)
End Function
I expect the function to allow me to open a workbook, then it will proceed to import the information and move forward with the code. The file selection box appears, but when I pick an excel file and hit OK, I receive the type mismatch error. Apologies for long winded text, still new to Stack Overflow. Thanks again!