False positive for cached package version

[JamieMason]

shrinkpack tried to read [email protected] from the cache...

! failed to read file /Users/jmason/.npm/object-assign/3.0.0/package.tgz
! failed to shrinkpack [email protected]
! Please raise an issue at https://github.com/JamieMason/shrinkpack/issues

    Error: ENOENT: no such file or directory, open '/Users/jmason/.npm/object-assign/3.0.0/package.tgz'
        at Error (native)

but it was not there, although other versions were.

$ npm cache ls | grep object-assign
~/.npm/object-assign/
~/.npm/object-assign/2.1.1/
~/.npm/object-assign/2.1.1/package.tgz
~/.npm/object-assign/2.1.1/package/
~/.npm/object-assign/2.1.1/package/package.json
~/.npm/object-assign/4.0.1/
~/.npm/object-assign/4.0.1/package.tgz
~/.npm/object-assign/4.0.1/package/
~/.npm/object-assign/4.0.1/package/package.json

[JamieMason]

This can happen when the package.json used to resolve the tarball does not match the version required.