diff --git a/debian/changelog b/debian/changelog
index dc4e4063..78aeed08 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,10 @@
+livecd-rootfs (2.509) UNRELEASED; urgency=medium
+
+  * Properly handle loop device names when there are > 10 devices on the
+    system.
+
+ -- Steve Langasek <steve.langasek@ubuntu.com>  Tue, 06 Mar 2018 10:30:02 +0100
+
 livecd-rootfs (2.508) bionic; urgency=medium
 
   * Use Ubuntu-specific branches when seeding snaps, as required in
diff --git a/live-build/functions b/live-build/functions
index 239169ad..66aadff1 100644
--- a/live-build/functions
+++ b/live-build/functions
@@ -56,7 +56,7 @@ mount_image() {
 
     # Find the loop device
     loop_p1="$(echo -e ${kpartx_mapping} | head -n1 | awk '{print$3}')"
-    loop_device="/dev/loop$(echo ${loop_p1} | cut -b5)"
+    loop_device="/dev/${loop_p1%p[0-9]*}"
     if [ ! -b ${loop_device} ]; then
         echo "unable to find loop device for ${backing_img}"
         exit 1