diff --git a/setupext.py b/setupext.py
index f43b5f92f2c1..727f67351d3a 100644
--- a/setupext.py
+++ b/setupext.py
@@ -906,7 +906,7 @@ def check(self):
             try:
                 check_include_file(get_include_dirs(), 'ft2build.h', 'freetype')
             except CheckFailed:
-                check_include_file(get_include_dirs(), 'freetype2\\ft2build.h', 'freetype')
+                check_include_file(get_include_dirs(), os.path.join('freetype2', 'ft2build.h'), 'freetype')
             return 'Using unknown version found on system.'
 
         status, output = subprocess.getstatusoutput(