diff --git a/server.c b/server.c
index 2da0c83..7ab01ef 100644
--- a/server.c
+++ b/server.c
@@ -955,7 +955,10 @@
 
 		stream.avail_out = FIO_SERVER_MAX_FRAGMENT_PDU;
 		stream.next_out = out_pdu;
-		assert(deflate(&stream, Z_FINISH) == Z_OK);
+		ret = deflate(&stream, Z_FINISH);
+		/* may be Z_OK, or Z_STREAM_END */
+		if (ret < 0)
+			goto err_zlib;
 
 		this_len = FIO_SERVER_MAX_FRAGMENT_PDU - stream.avail_out;