`ionic capacitor add ios` command fails with "Unable to find a specification for `GCDWebServer (~> 3.0)` depended upon by `Capacitor`"

Versions:

$ ionic --version                                                                                                                                                                                                                   
4.0.6

$ ng --version
     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/


Angular CLI: 6.1.3
Node: 9.10.0
OS: darwin x64
Angular: 6.1.2
... common, compiler, compiler-cli, core, forms, http
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.7.3
@angular-devkit/build-angular     0.7.3
@angular-devkit/build-optimizer   0.7.3
@angular-devkit/build-webpack     0.7.3
@angular-devkit/core              0.7.3
@angular-devkit/schematics        0.7.3
@angular/cli                      6.1.3
@ngtools/webpack                  6.1.3
@schematics/angular               0.7.3
@schematics/update                0.7.3
rxjs                              6.2.2
typescript                        2.7.2
webpack                           4.9.2

$ ruby --version
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]

Reproduction steps:

1. ionic start testing blank --type=angular
2. ionic build to generate www
3. cd ./testing
4. ionic capacitor add ios --verbose

Generates this error:

$ ionic capacitor add ios --verbose
  ionic:cli-utils Terminal info: { tty: true, ci: false } +0ms
  ionic:cli-utils CLI global options: { _: [ 'capacitor', 'add', 'ios' ], help: null, h: null, verbose: true, quiet: null, interactive: true, color: true, confirm: null, json: null, project: null, '--': [] } +3ms
  ionic:cli-utils:lib:project Project type from config: ionic/angular 4 (angular) +0ms
  ionic:cli-utils Project name: undefined +129ms
  ionic:cli Context: { binPath: '/Users/some-user/.nvm/versions/node/v9.10.0/lib/node_modules/ionic/bin/ionic', libPath: '/Users/some-user/.nvm/versions/node/v9.10.0/lib/node_modules/ionic', execPath: '/Users/some-user/git/Mercer/hive/mmds-ionic', version: '4.0.6' } +0ms
> capacitor add ios
✔ Installing iOS dependencies in 10.81s
✔ Adding native xcode project in: /Users/some-user/git/Mercer/hive/mmds-ionic/ios in 72.07ms
✔ add in 10.88s
✔ Copying web assets from www to ios/App/public in 388.02ms
✔ Copying native bridge in 50.04ms
✔ Copying capacitor.config.json in 2.58ms
✔ copy in 463.10ms
✔ Updating iOS plugins in 2.20ms
  Found 0 Capacitor plugins for ios:
✖ Updating iOS native dependencies:
✖ update ios:
[error] Analyzing dependencies
Fetching podspec for `Capacitor` from `../../node_modules/@capacitor/ios`
Fetching podspec for `CapacitorCordova` from `../../node_modules/@capacitor/ios`
[!] Unable to find a specification for `GCDWebServer (~> 3.0)` depended upon by `Capacitor`

[ERROR] An error occurred while running subprocess capacitor.

        capacitor add ios exited with exit code 1.

        Re-running this command with the --verbose flag may provide more information.
  ionic:cli-framework:utils:process onBeforeExit handler: process.exit received +0ms
  ionic:cli-framework:utils:process onBeforeExit handler: running 1 queued functions +0ms
  ionic:cli-framework:utils:process onBeforeExit handler: exiting (exit code 1) +113ms

[admosity]

Follow the instructions to install cocoapods here https://cocoapods.org/

sudo gem install cocoapods

[jcesarmobile]

Did you have CocoaPods installed before?

Can you run pod repo update and try to run the command again?