diff --git a/src/OT/Layout/GPOS/MarkBasePosFormat1.hh b/src/OT/Layout/GPOS/MarkBasePosFormat1.hh
index 73839a4c8..938369a50 100644
--- a/src/OT/Layout/GPOS/MarkBasePosFormat1.hh
+++ b/src/OT/Layout/GPOS/MarkBasePosFormat1.hh
@@ -122,6 +122,11 @@ struct MarkBasePosFormat1_2
     hb_ot_apply_context_t::skipping_iterator_t &skippy_iter = c->iter_input;
     skippy_iter.set_lookup_props (LookupFlag::IgnoreMarks);
 
+    if (c->last_base_until > buffer->idx)
+    {
+      c->last_base_until = 0;
+      c->last_base = -1;
+    }
     unsigned j;
     for (j = buffer->idx; j > c->last_base_until; j--)
     {
diff --git a/src/OT/Layout/GPOS/MarkLigPosFormat1.hh b/src/OT/Layout/GPOS/MarkLigPosFormat1.hh
index 447187102..7e7b438aa 100644
--- a/src/OT/Layout/GPOS/MarkLigPosFormat1.hh
+++ b/src/OT/Layout/GPOS/MarkLigPosFormat1.hh
@@ -104,6 +104,11 @@ struct MarkLigPosFormat1_2
     hb_ot_apply_context_t::skipping_iterator_t &skippy_iter = c->iter_input;
     skippy_iter.set_lookup_props (LookupFlag::IgnoreMarks);
 
+    if (c->last_base_until > buffer->idx)
+    {
+      c->last_base_until = 0;
+      c->last_base = -1;
+    }
     unsigned j;
     for (j = buffer->idx; j > c->last_base_until; j--)
     {
diff --git a/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-shape-fuzzer-6377756666757120 b/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-shape-fuzzer-6377756666757120
new file mode 100644
index 000000000..718e169a7
Binary files /dev/null and b/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-shape-fuzzer-6377756666757120 differ