diff --git a/drivers/acpi/proc.c b/drivers/acpi/proc.c
index d0d550d22a6d..f8b6f555ba52 100644
--- a/drivers/acpi/proc.c
+++ b/drivers/acpi/proc.c
@@ -398,6 +398,8 @@ acpi_system_write_wakeup_device(struct file *file,
 
 	if (len > 4)
 		len = 4;
+	if (len < 0)
+		return -EFAULT;
 
 	if (copy_from_user(strbuf, buffer, len))
 		return -EFAULT;