diff --git a/drivers/can/can.c b/drivers/can/can.c
index 4be8099fe7..cb6d7e27e7 100644
--- a/drivers/can/can.c
+++ b/drivers/can/can.c
@@ -811,7 +811,7 @@ static ssize_t can_write(FAR struct file *filep, FAR const char *buffer,
    * shorter than the minimum.
    */
 
-  while ((buflen - nsent) >= CAN_MSGLEN(0))
+  while (((ssize_t)buflen - nsent) >= CAN_MSGLEN(0))
     {
       /* Check if adding this new message would over-run the drivers ability
        * to enqueue xmit data.