diff --git a/code/IFCReaderGen1.cpp b/code/IFCReaderGen1.cpp
index d334e8322350441d104a438d9bd35ff9134f6041..d7753a841e732abdcbb6f55c091f598e585fe6b0 100644
--- a/code/IFCReaderGen1.cpp
+++ b/code/IFCReaderGen1.cpp
@@ -2652,6 +2652,7 @@ template <> size_t GenericFill<IfcNamedUnit>(const DB& db, const LIST& params, I
 	size_t base = 0;
 	if (params.GetSize() < 2) { throw STEP::TypeError("expected 2 arguments to IfcNamedUnit"); }    do { // convert the 'Dimensions' argument
         std::shared_ptr<const DataType> arg = params[base++];
+		if (dynamic_cast<const UNSET*>(&*arg)) break;
         if (dynamic_cast<const ISDERIVED*>(&*arg)) { in->ObjectHelper<Assimp::IFC::IfcNamedUnit,2>::aux_is_derived[0]=true; break; }
         try { GenericConvert( in->Dimensions, arg, db ); break; } 
         catch (const TypeError& t) { throw TypeError(t.what() + std::string(" - expected argument 0 to IfcNamedUnit to be a `IfcDimensionalExponents`")); }