diff --git a/src/hb-ot-layout-common.hh b/src/hb-ot-layout-common.hh
index 1799d4bd8..bb06ddb97 100644
--- a/src/hb-ot-layout-common.hh
+++ b/src/hb-ot-layout-common.hh
@@ -1218,8 +1218,8 @@ struct ClassDefFormat1
       return_trace (true);
     }
 
-    hb_codepoint_t glyph_min = glyphs[0];
-    hb_codepoint_t glyph_max = glyphs[glyphs.length - 1];
+    hb_codepoint_t glyph_min = hb_iter (glyphs) | hb_reduce (hb_min, 0xFFFFu);
+    hb_codepoint_t glyph_max = hb_iter (glyphs) | hb_reduce (hb_max, 0u);
 
     startGlyph = glyph_min;
     classValue.len = glyph_max - glyph_min + 1;
@@ -1510,8 +1510,8 @@ struct ClassDef
     unsigned int format = 2;
     if (likely (glyphs))
     {
-      hb_codepoint_t glyph_min = glyphs[0];
-      hb_codepoint_t glyph_max = glyphs[glyphs.length - 1];
+      hb_codepoint_t glyph_min = +glyphs | hb_reduce (hb_min, 0xFFFFu);
+      hb_codepoint_t glyph_max = +glyphs | hb_reduce (hb_max, 0u);
 
       unsigned int count = glyphs.len ();
       unsigned int num_ranges = 1;