Learn CSS Variables by Building a City Skyline - Step 40

says:
In .bb1a , add a gradient of type linear-gradient to the background property with --building-color1 as the first color and --window-color1 as the second.

before:

.bb1a {
  width: 70%;
  height: 10%;
  background-color: var(--building-color1);
}
after: 
.bb1a {
  width: 70%;
  height: 10%;
  background-color: var(--building-color1);
background: linear-gradient(--building-color1, --window-color1);
}

  **what is my mistake?

Link to the challenge:

You aren’t quite using the custom variables correctly. You are missing something. Look at the background-color property you added earlier to see how to use custom variables in CSS.

the answer is not explicit, but, I understood, thank you.