diff --git a/src/evdev.c b/src/evdev.c
index 5d01e3bc..ba7c8b34 100644
--- a/src/evdev.c
+++ b/src/evdev.c
@@ -715,8 +715,12 @@ err:
 int
 evdev_device_get_keys(struct evdev_device *device, char *keys, size_t size)
 {
+	int len;
+
 	memset(keys, 0, size);
-	return ioctl(device->fd, EVIOCGKEY(size), keys);
+	len = ioctl(device->fd, EVIOCGKEY(size), keys);
+
+	return (len == -1) ? -errno : len;
 }
 
 const char *
diff --git a/src/libinput.h b/src/libinput.h
index 9a9cd8c4..d6bf9f86 100644
--- a/src/libinput.h
+++ b/src/libinput.h
@@ -1169,6 +1169,8 @@ libinput_device_led_update(struct libinput_device *device,
  * @param device A current input device
  * @param keys An array filled with the bitmask for the keys
  * @param size Size of the keys array
+ *
+ * @return The number of valid bytes in keys, or a negative errno on failure
  */
 int
 libinput_device_get_keys(struct libinput_device *device,