diff --git a/src/mux_h2.c b/src/mux_h2.c
index fd039d1..74fffbf 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2475,6 +2475,12 @@
 			break;
 		}
 
+		if (h2s->st == H2_SS_IDLE && (h2c->flags & H2_CF_IS_BACK)) {
+			/* only PUSH_PROMISE would be permitted here */
+			h2c_error(h2c, H2_ERR_PROTOCOL_ERROR);
+			break;
+		}
+
 		if (h2s->st == H2_SS_HREM && h2c->dft != H2_FT_WINDOW_UPDATE &&
 		    h2c->dft != H2_FT_RST_STREAM && h2c->dft != H2_FT_PRIORITY) {
 			/* RFC7540#5.1: any frame other than WU/PRIO/RST in