diff --git a/src/hb-font-private.hh b/src/hb-font-private.hh
index c87ad5856..bdc6259e2 100644
--- a/src/hb-font-private.hh
+++ b/src/hb-font-private.hh
@@ -124,7 +124,7 @@ struct hb_face_t {
     return blob;
   }
 
-  inline unsigned int get_upem (void) const
+  inline HB_PURE_FUNC unsigned int get_upem (void) const
   {
     if (unlikely (!upem))
       load_upem ();