diff --git a/sd-image/sd-image.nix b/sd-image/sd-image.nix
index 60eedfc..b9fed6a 100644
--- a/sd-image/sd-image.nix
+++ b/sd-image/sd-image.nix
@@ -248,7 +248,7 @@ in
         # Figure out device names for the boot device and root filesystem.
         rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
         bootDevice=$(lsblk -npo PKNAME $rootPart)
-        partNum=$(lsblk -npo MAJ:MIN $rootPart | ${pkgs.gawk}/bin/awk -F: '{print $2}')
+        partNum=$(lsblk -npo PARTN $rootPart)
 
         # Resize the root partition and the filesystem to fit the disk
         echo ",+," | sfdisk -N$partNum --no-reread $bootDevice