ionic cordova plugin add error

**Description:**
ionic cordova plugin add cordova-plugin-googleplus --variable REVERSED_CLIENT_ID=myreversedclientid

**Output:**
(node:732) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): SyntaxError: Unexpected token A in JSON at position 0
(node:732) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

**My `ionic info`:**
cli packages: (C:\Users\soo\AppData\Roaming\npm\node_modules)

@ionic/cli-utils : 1.19.0
ionic (Ionic CLI) : 3.19.0

global packages:

cordova (Cordova CLI) : 8.0.0

local packages:

@ionic/app-scripts : 3.1.6
Cordova Platforms : none
Ionic Framework : ionic-angular 3.9.2

System:

Android SDK Tools : 26.1.1
Node : v8.9.0
npm : 5.6.0
OS : Windows 10

Environment Variables:

ANDROID_HOME : C:\Users\soo\AppData\Local\Android\Sdk

Misc:

backend : pro

No source file or test is named in the report. Reproduce the command using the listed Ionic, Cordova, Node, npm, and Windows versions, then trace the CLI's plugin-add error handling; done means the command no longer emits an unhandled JSON parsing rejection.

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