angular-cli: WARNING in '...' (Emitted Value instead of an instance of Error) postcss-url: foo/select.component.css can't read file 'bar.svg', ignoring

Versions

Angular CLI: 1.7.3
Node: 9.5.0
OS: darwin x64
Angular: 5.2.7
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

@angular/cdk: 5.2.3
@angular/cli: 1.7.3
@angular/material: 5.2.3
@angular-devkit/build-optimizer: 0.3.2
@angular-devkit/core: 0.3.2
@angular-devkit/schematics: 0.3.2
@ngtools/json-schema: 1.2.0
@schematics/angular: 0.3.2
@schematics/package-update: 0.3.2
typescript: 2.5.3
webpack: 3.11.0

Repro steps

• npm start

╰─ npm start

> prangular@0.0.0 start /Users/admin/code/prangular
> ng serve

** NG Live Development Server is listening on localhost:3200, open your browser on http://localhost:3200/ **
Date: 2018-03-18T22:07:30.793Z                                                - Hash: 8712ae5305fa8cf09699
Time: 13125ms
chunk {inline} inline.bundle.js (inline) 3.85 kB [entry] [rendered]
chunk {main} main.bundle.js (main) 318 kB [initial] [rendered]
chunk {polyfills} polyfills.bundle.js (polyfills) 549 kB [initial] [rendered]
chunk {styles} styles.bundle.js (styles) 441 kB [initial] [rendered]
chunk {vendor} vendor.bundle.js (vendor) 13.4 MB [initial] [rendered]

• Note: the resources that are failing are located in assets/images/*.*

Observed behavior

WARNING in ./src/app/atoms/form-elements/select/select.component.css
(Emitted value instead of an instance of Error) postcss-url: /Users/admin/code/prangular/src/app/atoms/form-elements/select/select.component.css:30:3: Can't read file '/Users/admin/code/prangular/src/app/atoms/form-elements/select/assets/images/down-arrow.svg', ignoring

WARNING in ./src/app/molecules/main-navigation/main-navigation.component.css
(Emitted value instead of an instance of Error) postcss-url: /Users/admin/code/prangular/src/app/molecules/main-navigation/main-navigation.component.css:40:3: Can't read file '/Users/admin/code/prangular/src/app/molecules/main-navigation/assets/images/pgr_logo_white.png', ignoring

One of the referenced images:

.pgr-logo {
  background-image: url(assets/images/pgr_logo_white.png);
  width: 169px;
  height: 34px;
  margin-left: 24px;
}

Desired behavior

No warnings?

Mention any other details that might be useful (optional)

This seems to happen for any file where I reference a background image in css.