"InvalidOperationException" is thrown when selecting a USS selector with a var() reference nested inside a gradient function

Steps to reproduce:

1. Create a new 3D URP project
2. Create a new UI Document and open it (Assets > Create > UI Toolkit > UI Document)
3. Under StyleSheets, click "+" and select "Create New USS"
4. Open the .uss file in an external code editor
5. Paste the following lines and Save:

:root { --my-col: rgb(20, 200, 90); }
.nested-f { background-image: linear-gradient(var(--my-col), blue); }

1. In UI Builder StyleSheets panel, select the ".nested-f" selector
2. Observe the Console

Actual results: "InvalidOperationException: Operation is not valid due to the current state of the object." error is thrown

Expected results: No errors

Reproducible with versions: 6000.7.0a4

Can’t test with versions: 6000.3.21f1, 6000.5.6f1, 6000.6.0b6, 6000.7.0a3 (Gradients feature not available in background-image)

Tested on (OS): macOS 26.5