btrfs-convert: Unable to find block group for 0

While trying to convert a ext4 filesystem to btrfs with the new btrfs-convert from btrfs-progs-4.6 I got the following error:

create btrfs filesystem:

        blocksize: 4096
        nodesize:  16384
        features:  extref, skinny-metadata (default)

creating ext2 image file.
Unable to find block group for 0
extent-tree.c:289: find_search_start: Assertion 1
failed. ./btrfs-convert(btrfs_reserve_extent+0x8e9)[0x41979d] ./btrfs-convert(btrfs_alloc_free_block+0x50)[0x419b27] ./btrfs-convert[0x40b8d8] ./btrfs-convert(btrfs_search_slot+0x13f7)[0x40d9d6] ./btrfs-convert(btrfs_csum_file_block+0x3b5)[0x41f144] ./btrfs-convert[0x4387b1] ./btrfs-convert(main+0x2013)[0x43d180] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7fca6bb895f0] ./btrfs-convert(_start+0x29)[0x4082b9]

Before I already tried btrfs-convert from btrfs-progs-4.5.2 which I accidentally aborted but until then seemed to run fine.