diff --git a/bin/varnishd/http2/cache_http2.h b/bin/varnishd/http2/cache_http2.h
index 11e957d499..95550c0807 100644
--- a/bin/varnishd/http2/cache_http2.h
+++ b/bin/varnishd/http2/cache_http2.h
@@ -163,7 +163,8 @@ struct h2_sess {
 
 	struct sess			*sess;
 	int				refcnt;
-	unsigned			open_streams;
+	int				open_streams;
+	int				winup_streams;
 	uint32_t			highest_stream;
 	int				goaway;
 	int				bogosity;
diff --git a/bin/varnishd/http2/cache_http2_send.c b/bin/varnishd/http2/cache_http2_send.c
index 39ddcfafd4..0d6b3a6e7b 100644
--- a/bin/varnishd/http2/cache_http2_send.c
+++ b/bin/varnishd/http2/cache_http2_send.c
@@ -267,6 +267,9 @@ h2_do_window(struct worker *wrk, struct h2_req *r2,
 		r2->t_winupd = VTIM_real();
 		h2_send_rel_locked(h2, r2);
 
+		assert(h2->winup_streams >= 0);
+		h2->winup_streams++;
+
 		while (r2->t_window <= 0 && h2_errcheck(r2, h2) == NULL) {
 			r2->cond = &wrk->cond;
 			(void)h2_cond_wait(r2->cond, h2, r2);
@@ -281,6 +284,10 @@ h2_do_window(struct worker *wrk, struct h2_req *r2,
 			h2_win_charge(r2, h2, w);
 			assert (w > 0);
 		}
+
+		assert(h2->winup_streams > 0);
+		h2->winup_streams--;
+
 		h2_send_get_locked(wrk, h2, r2);
 	}