diff --git a/src/hb-ot-shape-fallback.cc b/src/hb-ot-shape-fallback.cc
index 593c3335e..449b64e5c 100644
--- a/src/hb-ot-shape-fallback.cc
+++ b/src/hb-ot-shape-fallback.cc
@@ -430,41 +430,52 @@ _hb_ot_shape_fallback_kern (const hb_ot_shape_plan_t *plan,
 			    hb_font_t *font,
 			    hb_buffer_t  *buffer)
 {
-  unsigned int count = buffer->len;
   hb_mask_t kern_mask = plan->map.get_1_mask (HB_DIRECTION_IS_HORIZONTAL (buffer->props.direction) ?
 					      HB_TAG ('k','e','r','n') : HB_TAG ('v','k','r','n'));
+  if (!kern_mask) return;
+
+  unsigned int count = buffer->len;
 
   OT::hb_apply_context_t c (1, font, buffer);
   c.set_lookup_mask (kern_mask);
   c.set_lookup_props (OT::LookupFlag::IgnoreMarks);
 
-  for (buffer->idx = 0; buffer->idx < count;)
+  hb_glyph_info_t *info = buffer->info;
+  hb_glyph_position_t *pos = buffer->pos;
+
+  for (unsigned int idx = 0; idx < count;)
   {
-    OT::hb_apply_context_t::skipping_forward_iterator_t skippy_iter (&c, buffer->idx, 1);
+    OT::hb_apply_context_t::skipping_forward_iterator_t skippy_iter (&c, idx, 1);
     if (!skippy_iter.next ())
     {
-      buffer->idx++;
+      idx++;
       continue;
     }
 
-    hb_position_t x_kern, y_kern, kern1, kern2;
-    font->get_glyph_kerning_for_direction (buffer->info[buffer->idx].codepoint,
-					   buffer->info[skippy_iter.idx].codepoint,
+    hb_position_t x_kern, y_kern;
+    font->get_glyph_kerning_for_direction (info[idx].codepoint,
+					   info[skippy_iter.idx].codepoint,
 					   buffer->props.direction,
 					   &x_kern, &y_kern);
 
-    kern1 = x_kern >> 1;
-    kern2 = x_kern - kern1;
-    buffer->pos[buffer->idx].x_advance += kern1;
-    buffer->pos[skippy_iter.idx].x_advance += kern2;
-    buffer->pos[skippy_iter.idx].x_offset += kern2;
+    if (x_kern)
+    {
+      hb_position_t kern1 = x_kern >> 1;
+      hb_position_t kern2 = x_kern - kern1;
+      pos[idx].x_advance += kern1;
+      pos[skippy_iter.idx].x_advance += kern2;
+      pos[skippy_iter.idx].x_offset += kern2;
+    }
 
-    kern1 = y_kern >> 1;
-    kern2 = y_kern - kern1;
-    buffer->pos[buffer->idx].y_advance += kern1;
-    buffer->pos[skippy_iter.idx].y_advance += kern2;
-    buffer->pos[skippy_iter.idx].y_offset += kern2;
+    if (y_kern)
+    {
+      hb_position_t kern1 = y_kern >> 1;
+      hb_position_t kern2 = y_kern - kern1;
+      pos[idx].y_advance += kern1;
+      pos[skippy_iter.idx].y_advance += kern2;
+      pos[skippy_iter.idx].y_offset += kern2;
+    }
 
-    buffer->idx = skippy_iter.idx;
+    idx = skippy_iter.idx;
   }
 }