diff --git a/deflate.c b/deflate.c
index 3c302bc..1ab1635 100644
--- a/deflate.c
+++ b/deflate.c
@@ -1110,7 +1110,6 @@ int ZEXPORT deflate (strm, flush)
     }
 
     if (flush != Z_FINISH) return Z_OK;
-    if (s->wrap <= 0) return Z_STREAM_END;
 
     /* Write the trailer */
 #ifdef GZIP
@@ -1126,7 +1125,7 @@ int ZEXPORT deflate (strm, flush)
     }
     else
 #endif
-    {
+    if (s->wrap == 1) {
         putShortMSB(s, (uInt)(strm->adler >> 16));
         putShortMSB(s, (uInt)(strm->adler & 0xffff));
     }
@@ -1135,7 +1134,11 @@ int ZEXPORT deflate (strm, flush)
      * to flush the rest.
      */
     if (s->wrap > 0) s->wrap = -s->wrap; /* write the trailer only once! */
-    return s->pending != 0 ? Z_OK : Z_STREAM_END;
+    if (s->pending == 0) {
+        Assert(s->bi_valid == 0, "bi_buf not flushed");
+        return Z_STREAM_END;
+    }
+    return Z_OK;
 }
 
 /* ========================================================================= */