plugin add not woking

[dehghanimeh]

hi plugin add command not woking

$ cordova plugin add https://github.com/onsip/onsip-cordova.git

Error: Failed to fetch plugin https://github.com/onsip/onsip-cordova.git via registry. Probably this is either a connection problem, or plugin spec is incorrect. Check your connection and plugin name/version/URL. Error: npm: Command failed with exit code 1 Error output: npm ERR! cb() never called!

npm ERR! This is an error with npm itself. Please report this error at: npm ERR! https://github.com/npm/npm/issues

npm ERR! A complete log of this run can be found in: npm ERR! /Users/mehdi/.npm/_logs/2017-06-30T05_42_23_873Z-debug.log