diff --git a/libsolidity/experimental/analysis/TypeInference.cpp b/libsolidity/experimental/analysis/TypeInference.cpp
index 2e5a35853..d326d01ca 100644
--- a/libsolidity/experimental/analysis/TypeInference.cpp
+++ b/libsolidity/experimental/analysis/TypeInference.cpp
@@ -839,7 +839,7 @@ bool TypeInference::visit(TypeDefinition const& _typeDefinition)
 
 	TypeConstructor constructor = typeConstructor(&_typeDefinition);
 	auto [members, newlyInserted] = annotation().members.emplace(constructor, std::map<std::string, TypeMember>{});
-	solAssert(newlyInserted);
+	solAssert(newlyInserted, fmt::format("Members of type '{}' are already defined.", m_typeSystem.constructorInfo(constructor).name));
 	if (underlyingType)
 	{
 		members->second.emplace("abs", TypeMember{helper.functionType(*underlyingType, definedType)});
diff --git a/libsolidity/experimental/ast/TypeSystem.cpp b/libsolidity/experimental/ast/TypeSystem.cpp
index 0400f4196..717ecd4fe 100644
--- a/libsolidity/experimental/ast/TypeSystem.cpp
+++ b/libsolidity/experimental/ast/TypeSystem.cpp
@@ -297,7 +297,10 @@ experimental::Type TypeSystem::type(TypeConstructor _constructor, std::vector<Ty
 {
 	// TODO: proper error handling
 	auto const& info = m_typeConstructors.at(_constructor.m_index);
-	solAssert(info.arguments() == _arguments.size(), "Invalid arity.");
+	solAssert(
+		info.arguments() == _arguments.size(),
+		fmt::format("Type constructor '{}' accepts {} type arguments (got {}).", constructorInfo(_constructor).name, info.arguments(), _arguments.size())
+	);
 	return TypeConstant{_constructor, _arguments};
 }