diff --git a/src/tl/tl/tlDeflate.cc b/src/tl/tl/tlDeflate.cc
index 41f404b3c..b4a5ec04b 100644
--- a/src/tl/tl/tlDeflate.cc
+++ b/src/tl/tl/tlDeflate.cc
@@ -166,6 +166,7 @@ public:
       if (*l > 0) {
         unsigned int code = next_code [*l]++;
         code <<= (max_bits - *l);
+        tl_assert (code < (unsigned int) (1 << max_bits));
         mp_codes [code] = symbol;
         mp_bitmasks [code] = bitmasks [*l];
       }