diff --git a/drivers/lcd/pcf8574_lcd_backpack.c b/drivers/lcd/pcf8574_lcd_backpack.c
index 6f697d6600..5eaf874c8f 100644
--- a/drivers/lcd/pcf8574_lcd_backpack.c
+++ b/drivers/lcd/pcf8574_lcd_backpack.c
@@ -1064,6 +1064,7 @@ static int pcf8574_lcd_open(FAR struct file *filep)
   nxmutex_lock(&priv->lock);
   if (priv->refs == MAX_OPENCNT)
     {
+      nxmutex_unlock(&priv->lock);
       return -EMFILE;
     }
   else