diff --git a/src/lib-compression/ostream-zlib.c b/src/lib-compression/ostream-zlib.c
index 2851762de6..b2b3bde338 100644
--- a/src/lib-compression/ostream-zlib.c
+++ b/src/lib-compression/ostream-zlib.c
@@ -32,7 +32,8 @@ static void o_stream_zlib_close(struct iostream_private *stream,
 {
 	struct zlib_ostream *zstream = (struct zlib_ostream *)stream;
 
-	(void)o_stream_flush(&zstream->ostream.ostream);
+	i_assert(zstream->ostream.finished ||
+		 zstream->ostream.ostream.stream_errno != 0);
 	(void)deflateEnd(&zstream->zs);
 	if (close_parent)
 		o_stream_close(zstream->ostream.parent);
@@ -230,7 +231,7 @@ static int o_stream_zlib_flush(struct ostream_private *stream)
 	struct zlib_ostream *zstream = (struct zlib_ostream *)stream;
 	int ret;
 
-	if (o_stream_zlib_send_flush(zstream, TRUE) < 0)
+	if (o_stream_zlib_send_flush(zstream, stream->finished) < 0)
 		return -1;
 
 	ret = o_stream_flush(stream->parent);
diff --git a/src/lib-compression/test-compression.c b/src/lib-compression/test-compression.c
index ff1658c7bf..5dbe6db94f 100644
--- a/src/lib-compression/test-compression.c
+++ b/src/lib-compression/test-compression.c
@@ -62,6 +62,7 @@ static void test_compression_handler(const struct compression_handler *handler)
 		test_assert(o_stream_send(output, buf, sizeof(buf)) == sizeof(buf));
 	}
 
+	test_assert(o_stream_finish(output) > 0);
 	o_stream_destroy(&output);
 	o_stream_destroy(&file_output);
 	sha1_result(&sha1, output_sha1);