diff --git a/llvm/include/llvm/Cheerp/TypeOptimizer.h b/llvm/include/llvm/Cheerp/TypeOptimizer.h
index d02859074ed1..c916cb1c0b7a 100644
--- a/llvm/include/llvm/Cheerp/TypeOptimizer.h
+++ b/llvm/include/llvm/Cheerp/TypeOptimizer.h
@@ -120,6 +120,7 @@ class TypeOptimizer
 
 	llvm::Module* module;
 	const llvm::DataLayout* DL;
+	std::unordered_set<const llvm::Type*> uncollapsibleSecondaryBases;
 	std::unordered_map<const llvm::StructType*,std::set<llvm::StructType*>> downcastSourceToDestinationsMapping;
 	std::unordered_map<const llvm::StructType*, std::vector<std::pair<uint32_t, uint32_t>>> membersMappingData;
 	std::unordered_map<llvm::GlobalValue*, llvm::Constant*> globalsMapping;
diff --git a/llvm/lib/CheerpUtils/TypeOptimizer.cpp b/llvm/lib/CheerpUtils/TypeOptimizer.cpp
index b89b94ac861e..0d9bb1099d6a 100644
--- a/llvm/lib/CheerpUtils/TypeOptimizer.cpp
+++ b/llvm/lib/CheerpUtils/TypeOptimizer.cpp
@@ -263,6 +263,21 @@ void TypeOptimizer::gatherAllTypesInfo(const Module& M)
 		uint32_t fieldIndex = cast<ConstantInt>(*std::prev(GEP->op_end()))->getZExtValue();
 		escapingFields.emplace(containerStructType, fieldIndex, TypeAndIndex::STRUCT_MEMBER);
 	}
+
+	// Mark classes that cannot be collapsed because they are used as a secondary base
+	for (const auto sTy : M.getIdentifiedStructTypes())
+	{
+		uint32_t firstBase;
+		uint32_t baseCount;
+		if (TypeSupport::getBasesInfo(M, sTy, firstBase, baseCount))
+		{
+			for (uint32_t i = firstBase; i< (firstBase + baseCount); i++)
+			{
+				assert(sTy->getElementType(i)->isStructTy());
+				uncollapsibleSecondaryBases.insert(sTy->getElementType(i));
+			}
+		}
+	}
 }
 
 /**
@@ -294,6 +309,9 @@ bool TypeOptimizer::canCollapseStruct(llvm::StructType* st, llvm::StructType* ne
 		assert(st->isLiteral());
 		return true;
 	}
+	// Secondary bases used inside a hierarchy where a downcast is performed on cannot safely be collapsed
+	if (uncollapsibleSecondaryBases.count(st))
+		return false;
 	// Stop if the element is just a int8, we may be dealing with an empty struct
 	// Empty structs are unsafe as the int8 inside is just a placeholder and will be replaced
 	// by a different type in a derived class