lacks a valid hash

Hi everyone. I follow the example in the https://raw.githubusercontent.com/nfeske/genode-world/master/doc/numptyphysi... port file with the following content: LICENSE := GPLv3 VERSION := git DOWNLOADS := numptyphysics.git

URL(numptyphysics) := https://github.com/thp/numptyphysics.git DIR(numptyphysics) := src/app/numptyphysics REV(numptyphysics) := 336729c44c9ced3907d049cc1c1130a26c5708a3 hash file with following content： dummy

when i use prepare _port tool: <genode-dir>/tool/prepare_port numptyphysics CHECK_HASH=no with following errors: Error: Port numptyphysics lacks a valid hash make: *** [nonexisting_hash] Error 1 why i use argument “CHECK_HASH=no”?but it still report the error.