diff --git a/src/mux_quic.c b/src/mux_quic.c
index 98737367e..fe33e4d51 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -1436,6 +1436,7 @@ static int qcs_build_stream_frm(struct qcs *qcs, struct buffer *out, char fin,
 	frm->stream.id = qcs->id;
 	frm->stream.buf = out;
 	frm->stream.data = (unsigned char *)b_peek(out, head);
+	frm->stream.offset.key = 0;
 
 	/* FIN is positioned only when the buffer has been totally emptied. */
 	if (fin)
diff --git a/src/quic_conn.c b/src/quic_conn.c
index 26a75f563..a64cd654d 100644
--- a/src/quic_conn.c
+++ b/src/quic_conn.c
@@ -6857,8 +6857,7 @@ static inline int qc_build_frms(struct list *outlist, struct list *inlist,
 				new_cf->stream.stream = cf->stream.stream;
 				new_cf->stream.buf = cf->stream.buf;
 				new_cf->stream.id = cf->stream.id;
-				if (cf->type & QUIC_STREAM_FRAME_TYPE_OFF_BIT)
-					new_cf->stream.offset = cf->stream.offset;
+				new_cf->stream.offset = cf->stream.offset;
 				new_cf->stream.len = dlen;
 				new_cf->type |= QUIC_STREAM_FRAME_TYPE_LEN_BIT;
 				/* FIN bit reset */
diff --git a/src/quic_frame.c b/src/quic_frame.c
index 6f299ba2d..0d8bc87f5 100644
--- a/src/quic_frame.c
+++ b/src/quic_frame.c
@@ -507,6 +507,10 @@ static int quic_build_stream_frame(unsigned char **buf, const unsigned char *end
 	struct quic_stream *stream = &frm->stream;
 	const unsigned char *wrap;
 
+	/* Caller must set OFF bit if and only if a non-null offset is used. */
+	BUG_ON(!!(frm->type & QUIC_STREAM_FRAME_TYPE_OFF_BIT) !=
+	       !!stream->offset.key);
+
 	if (!quic_enc_int(buf, end, stream->id) ||
 	    ((frm->type & QUIC_STREAM_FRAME_TYPE_OFF_BIT) && !quic_enc_int(buf, end, stream->offset.key)) ||
 	    ((frm->type & QUIC_STREAM_FRAME_TYPE_LEN_BIT) &&