diff --git a/src/h3.c b/src/h3.c
index 1006813c5..9f7bb0206 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -2179,9 +2179,14 @@ static size_t h3_nego_ff(struct qcs *qcs, size_t count)
 	return ret;
 }
 
+/* Finalize encoding of a HTTP/3 data frame after zero-copy for <qcs> stream.
+ * No frame is encoded if <qcs> iobuf indicates that no data were transferred.
+ *
+ * Return the payload size of the H3 data frame or 0 if no frame encoded.
+ */
 static size_t h3_done_ff(struct qcs *qcs)
 {
-	size_t total = qcs->sd->iobuf.data;
+	size_t total = 0;
 	TRACE_ENTER(H3_EV_STRM_SEND, qcs->qcc->conn, qcs);
 
 	h3_debug_printf(stderr, "%s\n", __func__);
@@ -2189,15 +2194,14 @@ static size_t h3_done_ff(struct qcs *qcs)
 	if (qcs->sd->iobuf.data) {
 		TRACE_DATA("encoding DATA frame (fast forward)",
 		           H3_EV_TX_FRAME|H3_EV_TX_DATA, qcs->qcc->conn, qcs);
+
 		b_sub(qcs->sd->iobuf.buf, qcs->sd->iobuf.data);
 		b_putchr(qcs->sd->iobuf.buf, 0x00); /* h3 frame type = DATA */
 		b_quic_enc_int(qcs->sd->iobuf.buf, qcs->sd->iobuf.data, QUIC_VARINT_MAX_SIZE); /* h3 frame length */
 		b_add(qcs->sd->iobuf.buf, qcs->sd->iobuf.data);
-	}
 
-	qcs->sd->iobuf.buf = NULL;
-	qcs->sd->iobuf.offset = 0;
-	qcs->sd->iobuf.data = 0;
+		total = qcs->sd->iobuf.offset + qcs->sd->iobuf.data;
+	}
 
 	TRACE_LEAVE(H3_EV_STRM_SEND, qcs->qcc->conn, qcs);
 	return total;
diff --git a/src/hq_interop.c b/src/hq_interop.c
index d0c6b43c2..18e9e2ccc 100644
--- a/src/hq_interop.c
+++ b/src/hq_interop.c
@@ -214,14 +214,8 @@ static size_t hq_interop_nego_ff(struct qcs *qcs, size_t count)
 
 static size_t hq_interop_done_ff(struct qcs *qcs)
 {
-	const size_t ret = qcs->sd->iobuf.data;
-
-	/* No header required for HTTP/0.9, simply mark ff as done. */
-	qcs->sd->iobuf.buf = NULL;
-	qcs->sd->iobuf.offset = 0;
-	qcs->sd->iobuf.data = 0;
-
-	return ret;
+	/* No header required for HTTP/0.9. */
+	return qcs->sd->iobuf.data;
 }
 
 static int hq_interop_attach(struct qcs *qcs, void *conn_ctx)
diff --git a/src/mux_quic.c b/src/mux_quic.c
index a06b36ddf..24e71ab92 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -3385,12 +3385,14 @@ static size_t qmux_strm_done_ff(struct stconn *sc)
 		goto end;
 	}
 
-	if (data)
-		data += sd->iobuf.offset;
 	total = qcs->qcc->app_ops->done_ff(qcs);
+	if (total || qcs->flags & QC_SF_FIN_STREAM)
+		qcc_send_stream(qcs, 0, total);
 
-	if (data || qcs->flags & QC_SF_FIN_STREAM)
-		qcc_send_stream(qcs, 0, data);
+	/* Reset stconn iobuf information. */
+	qcs->sd->iobuf.buf = NULL;
+	qcs->sd->iobuf.offset = 0;
+	qcs->sd->iobuf.data = 0;
 
 	/* Similar to snd_buf callback. */
 	if (!(qcs->qcc->wait_event.events & SUB_RETRY_SEND))
@@ -3400,7 +3402,7 @@ static size_t qmux_strm_done_ff(struct stconn *sc)
 
   end:
 	TRACE_LEAVE(QMUX_EV_STRM_SEND, qcs->qcc->conn, qcs);
-	return total;
+	return data;
 }
 
 static int qmux_strm_resume_ff(struct stconn *sc, unsigned int flags)