diff --git a/src/boot.sh b/src/boot.sh
index 1fb847b..0f8d8fd 100644
--- a/src/boot.sh
+++ b/src/boot.sh
@@ -100,10 +100,11 @@ else
 fi
 
 SM_BIOS=""
+PS="/sys/class/dmi/id/product_serial"
 
-if [ -s "/sys/class/dmi/id/product_serial" ]; then
+if [ -s "$PS" ] && [ -r "$PS" ]; then
 
-  BIOS_SERIAL=$(</sys/class/dmi/id/product_serial)
+  BIOS_SERIAL=$(<"$PS")
   BIOS_SERIAL="${BIOS_SERIAL//[![:alnum:]]/}"
 
   if [ -n "$BIOS_SERIAL" ]; then