Call of sub type failiure, how do i register/check it?

I've writen some code in the solidworks api and i have a line i wish to check if it goes through.
swModel.InsertOffsetSurface 0.01, False
it makes an offset surface on the face im currently on, but sometimes it is not posible and i get no indication of it. How do i check if it went through?
the code around it looks like this:
Do While Not swFace Is Nothing Set swEnt = swFace 'Select through Entity bRet = swEnt.Select4(True, swSelData): Debug.Assert bRet swModel.InsertOffsetSurface 0.01, False swModel.ClearSelection Set swFace = swFace.GetNextFace Loop
Thanks in advance!
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