diff --git a/projects/openttd_vs90.vcproj b/projects/openttd_vs90.vcproj
--- a/projects/openttd_vs90.vcproj
+++ b/projects/openttd_vs90.vcproj
@@ -1696,6 +1696,10 @@
 				RelativePath=".\..\src\core\sort_func.hpp"
 				>
 			</File>
+			<File
+				RelativePath=".\..\src\core\string_compare_type.hpp"
+				>
+			</File>
 		</Filter>
 		<Filter
 			Name="GUI Source Code"