lacks a valid hash

[missss]

Hi everyone.
  I follow  the example in the https://raw.githubusercontent.com/nfeske/genode-world/master/doc/numptyphysics.txt
   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.

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