functions with Dim as Double

I have a function in which I pull in cost and dnet and want to do multiplication with it. For some reason this doesn't work when I change Cost as string and DNET as string to Cost as double and DNET as double.

Public Function Create_REF(Cost As Double, DNET As Double) As String
If Cost <> "" Then
    If DNET <> "" Then
        Create_REF = Cost * DNET
    Else
    Create_REF = ""
    End If
Else
Create_REF = ""
End If
End Function