diff --git a/hw/char/cadence_uart.c b/hw/char/cadence_uart.c
index c856fc30b2a8..844542fd5ab5 100644
--- a/hw/char/cadence_uart.c
+++ b/hw/char/cadence_uart.c
@@ -288,8 +288,11 @@ static gboolean cadence_uart_xmit(GIOChannel *chan, GIOCondition cond,
     }
 
     ret = qemu_chr_fe_write(s->chr, s->tx_fifo, s->tx_count);
-    s->tx_count -= ret;
-    memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
+
+    if (ret >= 0) {
+        s->tx_count -= ret;
+        memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
+    }
 
     if (s->tx_count) {
         int r = qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,