diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index 44fbe582e..ed45374b7 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -224,7 +224,7 @@ HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS_SIMPLE
     switch (u)
     {
       /* All GC=Zs chars that can use a fallback. */
-      default:	    return NOT_SPACE;	/* Shouldn't happen. */
+      default:	    return NOT_SPACE;	/* U+1680 OGHAM SPACE MARK */
       case 0x0020u: return SPACE;	/* U+0020 SPACE */
       case 0x00A0u: return SPACE;	/* U+00A0 NO-BREAK SPACE */
       case 0x2000u: return SPACE_EM_2;	/* U+2000 EN QUAD */