Description:

• A-Frame Version: 1.4.1
• Platform / Device: Chrome Desktop
• Reproducible Code Snippet or URL:

When defining a gltf-model using url scheme as a mixin, the model does not load. However when using the same syntax without a mixin, the file loads.

      <a-assets>
        <a-mixin id="fire-truck-rig" gltf-model="url(https://assets.3dstreet.app/sets/vehicles-rig/gltf-exports/draco/fire-truck-pumper-rig.glb)"></a-mixin>
      </a-assets>
      <a-entity position="0 0 -9" mixin="fire-truck-rig"></a-entity>

generates console error of:

three.module.js:42746 Uncaught (in promise) TypeError: e.lastIndexOf is not a function
    at nd.extractUrlBase (three.module.js:42746:21)
    at pm.GLTFLoader.load (GLTFLoader.js:174:31)
    at gltf-model.js:44:19
e

Whereas this same syntax without using a mixin works as expected:
<a-entity gltf-model="url(https://assets.3dstreet.app/sets/vehicles-rig/gltf-exports/draco/fire-truck-pumper-rig.glb)"></a-entity>

reproducible here: https://mixin-gltf-url-error.glitch.me/
source: https://glitch.com/edit/#!/mixin-gltf-url-error