diff --git a/CHANGELOG.md b/CHANGELOG.md
index a36e22a0..9fb78681 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@ Version 78:
 * Add span
 * Documentation work
 * Use make_unique_noinit
+* Fix warning in zlib
 
 HTTP:
 
diff --git a/include/beast/zlib/detail/deflate_stream.hpp b/include/beast/zlib/detail/deflate_stream.hpp
index 9ec35be7..b51baff1 100644
--- a/include/beast/zlib/detail/deflate_stream.hpp
+++ b/include/beast/zlib/detail/deflate_stream.hpp
@@ -1059,8 +1059,9 @@ doWrite(z_params& zs, boost::optional<Flush> flush, error_code& ec)
             return;
         }
     }
-    else if(zs.avail_in == 0 && flush <= old_flush &&
-        flush != Flush::finish)
+    else if(zs.avail_in == 0 && (
+            old_flush && flush <= *old_flush
+        ) && flush != Flush::finish)
     {
         /* Make sure there is something to do and avoid duplicate consecutive
          * flushes. For repeated and useless calls with Flush::finish, we keep