diff --git a/src/ObjLoading/Parsing/Menu/Matcher/MenuExpressionMatchers.cpp b/src/ObjLoading/Parsing/Menu/Matcher/MenuExpressionMatchers.cpp
index 684257b1a..9ec73c373 100644
--- a/src/ObjLoading/Parsing/Menu/Matcher/MenuExpressionMatchers.cpp
+++ b/src/ObjLoading/Parsing/Menu/Matcher/MenuExpressionMatchers.cpp
@@ -92,6 +92,7 @@ std::unique_ptr<ISimpleExpression> MenuExpressionMatchers::ProcessOperandExtensi
 
     const auto& functionCallToken = result.NextCapture(CAPTURE_FUNCTION_NAME);
     auto functionCallName = functionCallToken.IdentifierValue();
+    utils::MakeStringLowerCase(functionCallName);
 
     const auto& baseFunctionMap = GetBaseFunctionMapForFeatureLevel(m_state->m_feature_level);
     const auto foundBaseFunction = baseFunctionMap.find(functionCallName);
diff --git a/src/Utils/Utils/StringUtils.cpp b/src/Utils/Utils/StringUtils.cpp
index cffa5ed61..bf8904da9 100644
--- a/src/Utils/Utils/StringUtils.cpp
+++ b/src/Utils/Utils/StringUtils.cpp
@@ -92,12 +92,12 @@ namespace utils
     void MakeStringLowerCase(std::string& str)
     {
         for (auto& c : str)
-            c = static_cast<char>(tolower(c));
+            c = static_cast<char>(tolower(static_cast<unsigned char>(c)));
     }
 
     void MakeStringUpperCase(std::string& str)
     {
         for (auto& c : str)
-            c = static_cast<char>(toupper(c));
+            c = static_cast<char>(toupper(static_cast<unsigned char>(c)));
     }
 } // namespace utils