diff --git a/src/hb-cff-interp-common.hh b/src/hb-cff-interp-common.hh
index f2ccc2b..9b595ff 100644
--- a/src/hb-cff-interp-common.hh
+++ b/src/hb-cff-interp-common.hh
@@ -391,8 +391,22 @@
 
   inline operator ByteStr (void) const { return ByteStr (str, offset, str.len - offset); }
 
-  inline bool avail (unsigned int count=1) const { return str.check_limit (offset, count); }
-  inline void inc (unsigned int count=1) { offset += count; assert (count <= str.len); }
+  inline bool avail (unsigned int count=1) const
+  {
+    return (!in_error () && str.check_limit (offset, count));
+  }
+  inline void inc (unsigned int count=1)
+  {
+    if (likely (!in_error () && (offset <= str.len) && (offset + count <= str.len)))
+    {
+      offset += count;
+    }
+    else
+    {
+      offset = str.len;
+      set_error ();
+    }
+  }
 
   inline void set_error (void) { error = true; }
   inline bool in_error (void) const { return error; }
diff --git a/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-subset-fuzzer-5768186323009536 b/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-subset-fuzzer-5768186323009536
new file mode 100644
index 0000000..858604d
--- /dev/null
+++ b/test/fuzzing/fonts/clusterfuzz-testcase-minimized-hb-subset-fuzzer-5768186323009536
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