FCollada string bug?

Aybody here use FCollada? I’m trying to track down an apparent bug with fstring, but I can’t seem to find it.
The bug happens when FCDGeometryInstance::AddMaterialInstance() is called. This line here:

fstring semantic = TO_FSTRING(material->GetDaeId()) + TO_FSTRING(polygons->GetFaceOffset());

GetDaeId() returns a string, like, “VisualMaterial” and GetFaceOffset() returns a number, like “1”,
so the final string should be “VisualMaterial1”. But instead, I get “VisualMaterial1VisualMaterial1”,
as if the + operator or something is concatentating the string twice.

I’m not quite sure what is happening.