I get error message of "Function argument value, type, or count is invalid"

I have a Combobox (This.Value) with the following:
ContorlSource: auth.authtype
RowSource: authtype.authtype
RowSourceType: 6 - Fields

and I want to check to see if the data (This.Value) matches the table (authtype.authtype) because the other person could do something that I have no idea how he did it (spaces or leaving the field and then come back and do something), so I want to do something like the following:

IF NOT INDEXSEEK(This.Value,authtype.authtype)
	RETURN .F.
ENDIF

Yet, does not work (checked the debug and both are the same field type as well), any advices?

Chuck