Install ubuntu to luks encrypted partition using preseed

Question

I have made luks encrypted partition as suggested in How can I install Ubuntu encrypted with LUKS with dual-boot? .

Now I am trying to install ubuntu using preseed file. Encrypted partition is in /dev/sda7. Below is snapshot of partman-auto/expert_recipe below.

d-i partman-auto/expert_recipe string root-disk ::  \
    100% 1024 100% ext4                         \
        $primary{ } $bootable{ }                \
        method{ format } format{ }              \
        device{ /dev/sda6 }                     \
        use_filesystem{ } filesystem{ ext2 }    \
        mountpoint{ /boot }                     \
        .                                       \
    100% 10240 100% ext4                         \
        $lvmok{ }                               \
        in_vg { vgRoot } lv_name{ lvRoot}   \
        device{ /dev/sda7 }                     \
        method{ format } format{ }              \
        use_filesystem{ } filesystem{ ext4 }    \
        mountpoint{ / }                         \

When I am trying install, it is taking me to partman page with error device /dev/mapper/vgRoot-lvRoot not found in os prober output. Can someone help with this error.