diff --git a/source/common/searchpaths.cpp b/source/common/searchpaths.cpp
index 4b2a53cd4..7afb7ed0b 100644
--- a/source/common/searchpaths.cpp
+++ b/source/common/searchpaths.cpp
@@ -678,9 +678,9 @@ static bool CheckAddon(GrpInfo* addon, GrpInfo* main, const char* filename)
 		FString check = path + fn;
 		if (FileExists(check)) continue;
 #if !defined _WIN32 
-		check = check.MakeLower();
+		check = path + fn.MakeLower();
 		if (FileExists(check)) continue;
-		check = check.MakeUpper();
+		check = path + fn.MakeUpper();
 		if (FileExists(check)) continue;
 #endif
 		return false;