diff --git a/drivers/accessibility/speakup/synth.c b/drivers/accessibility/speakup/synth.c
index ac47dbac7207..82cfc5ec6bdf 100644
--- a/drivers/accessibility/speakup/synth.c
+++ b/drivers/accessibility/speakup/synth.c
@@ -208,8 +208,10 @@ void spk_do_flush(void)
 	wake_up_process(speakup_task);
 }
 
-void synth_write(const char *buf, size_t count)
+void synth_write(const char *_buf, size_t count)
 {
+	const unsigned char *buf = (const unsigned char *) _buf;
+
 	while (count--)
 		synth_buffer_add(*buf++);
 	synth_start();