diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0c38017..f0e074a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -144,7 +144,7 @@ if(HAVE_STRNDUP)
 endif()
 
 if(MSVC)
-	set(HAVE_STRNLEN)
+	set(HAVE_STRNLEN true)
 	add_definitions(-DHAVE_STRNLEN)
 else()
 	check_function_exists(strnlen HAVE_STRNLEN)