diff --git a/src/core/application.cpp b/src/core/application.cpp
index 579aa38d..3557cdfa 100644
--- a/src/core/application.cpp
+++ b/src/core/application.cpp
@@ -4838,7 +4838,7 @@ void Application::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
                     if (basePath.size() > 4) {
                         extension = basePath.substr(basePath.size() - 4);
                         std::string extLower = extension;
-                        for (char& c : extLower) c = std::tolower(c);
+                        for (char& c : extLower) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
                         if (extLower == ".wmo") {
                             basePath = basePath.substr(0, basePath.size() - 4);
                         }
@@ -4949,7 +4949,7 @@ void Application::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
 
                                 if (m2Path.size() > 4) {
                                     std::string ext = m2Path.substr(m2Path.size() - 4);
-                                    for (char& c : ext) c = std::tolower(c);
+                                    for (char& c : ext) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
                                     if (ext == ".mdx" || ext == ".mdl") {
                                         m2Path = m2Path.substr(0, m2Path.size() - 4) + ".m2";
                                     }
@@ -5789,7 +5789,7 @@ void Application::buildGameObjectDisplayLookups() {
             if (modelName.empty()) continue;
             if (modelName.size() >= 4) {
                 std::string ext = modelName.substr(modelName.size() - 4);
-                for (char& c : ext) c = static_cast<char>(std::tolower(c));
+                for (char& c : ext) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
                 if (ext == ".mdx") {
                     modelName = modelName.substr(0, modelName.size() - 4) + ".m2";
                 }