I don't understand what's wrong with Less?

Question

css

This error appears when writing a less file:
"JavaScript evaluation error: Refused to evaluate a string as JavaScript because 'unsafe-eval' is not an allowed source of script in the following Content Security Policy directive: "script-src 'self'". from `"'../img/sprite.png'".slice(1, -1)`"
The code itself: codepen.io/anon/pen/peMQrB
Everything works as it should, but the error does not disappear