Modules & VBA :: Testing For Out Of Context Errors

I have quite a simple function. I am calling it in a query (through the Build window) like this:

Code : AbsMag: getAbsoluteMagnitude([Tbl]![m],[Tbl]![Distance])

When the input value for m is empty/Null, the response returned in the query is #Error. Problem is, any way I try to test for this in the function, it still returns #Error.The Watch for m reads "<Out of Context>" for these errors. Anyway to test/trap for this within the function itself?

Code:
Public Function getAbsoluteMagnitude(m As Double, d As Double, Optional dUnit As String = "Mpc") As Double
Dim absoluteMag As Double
' IsError(m) returns False
' IsNull(m) returns False
' IsEmpty(m) returns False

[code]...