Undefined Function In Expression

I created a function and when I try to use it in my query I am getting the error message Undefined Function in Expression.

This is the function:

Option Compare Database
Option Explicit

Function Next_Weekday(DateField As Date) As Date

If Next_Weekday((DateField)) = 1 Then
Next_Weekday = (DateField) + 1
Else
If Next_Weekday((DateField)) = 7 Then
Next_Weekday = (DateField) + 2
Else
Next_Weekday
End If

End Function

Any suggestions as to why it won't recognize it?

Thanks.