--import-blocks does not work

[cmichi]

The following happens on master:

$ cargo run --release -- export-blocks --to 1000 > 1k-blocks.bin

$ cargo run --release -- import-blocks 1k-blocks.bin
2019-02-14 10:32:53 Importing 1000 blocks
2019-02-14 10:32:53 Error reading block data at 0.
2019-02-14 10:32:53 Imported 0 blocks. Best: #1896

I always run into Error reading block data at 0.. It also doesn't change anything if I export as JSON. Piping the file into the process makes no difference either.

Also tried cargo clean, a cargo run --release -- purge-chain between export/import, etc.. The database also contains enough blocks.

The import always fails at a None returned here: https://github.com/paritytech/substrate/blob/master/core/service/src/chain_ops.rs#L127.