Error on line starting with ">"

[sherrybomb]

I'm using 1.17.1 and getting this error

ERROR 10642:5 no viable alternative at input '>' in ruleset (which started at 10624:1)

on the line marked below:

// ...
.xUiForm.xFillSpace {
    > .xUiFormResGrid {
        height: 100%;
        > .xUiFormTitle {
            flex: 0 0 auto;
        }
        > .xUiRespGrid {
            height: 100%;
            [class*="xUiRespGridSpan"]:not(.xUiFormElementLbl) {
                min-height: 2.75rem;
            }
        }
    }    
     > .xUiFormResGrid > .xUiRGLContainer {  // <----- error
        height: 100%;
        > .xUiFormContainerTitle {
            flex: 0 0 auto;
        }
        > .xUiRGLContainerCont {
            flex: 1 1 auto;
            > .xUiRespGrid {
                .xFillSpace(@width: ~"calc(100% + "@formResponsiveGridLayoutGutterWidth/2~")");  // <-- actual problematic line
                display:flex;
        // ...

This does not cause a problem with lessc. Changing the problematic line to this resolves the error:

.xFillSpace(@width: ~"calc(100% + @{formResponsiveGridLayoutHalfGutterWidth})");

[edited after initial post with more info]