Why am I getting a "expected '}' but found '{'" error in dotless?

Question

I have tried to append the values using +_ in less, but it throws the error

expected '}' but found '{'

in the dotless compiler. What did I miss?

.gradient(@brow){      
    @{brow}+_: ~"; background: @{brow}linear-gradient(top";
     // some content
  }
}

The error indicates in this lineL @{brow}+_:.