Compile fails when class selector includes what appears to be a calculation

[ghost]

There appears to be a problem when a class selector includes a class name which has a segment which appears to be a unit or calculation. Parsing Sass which includes Font Awesome the following selectors cause Exceptions to be thrown:

• .#{$fa-css-prefix}-500px (see line 661)

  Apr 04, 2016 3:28:29 PM com.vaadin.sass.internal.handler.SCSSErrorHandler log
  SEVERE: Error when parsing file 
  /PATH/fa/_icons.scss on line 661, column 19
  Apr 04, 2016 3:28:29 PM com.vaadin.sass.internal.handler.SCSSErrorHandler log
  SEVERE: encountered "500px". Was expecting one of: "-" "," "[" "%" "." "and" "or" "not" ":" "#{" "through" "in" "{" <IDENT> <HASH> 
  org.w3c.css.sac.CSSParseException: encountered "500px". Was expecting one of: "-" "," "[" "%" "." "and" "or" "not" ":" "#{" "through" "in" "{" <IDENT> <HASH> 
  at com.vaadin.sass.internal.parser.Parser.reportError(Parser.java:418)
  at com.vaadin.sass.internal.parser.Parser.styleRule(Parser.java:2203)
  at com.vaadin.sass.internal.parser.Parser.topLevelDeclaration(Parser.java:591)
  at com.vaadin.sass.internal.parser.Parser.parserUnit(Parser.java:487)
  at com.vaadin.sass.internal.parser.Parser.parseStyleSheet(Parser.java:122)
  at com.vaadin.sass.internal.ScssStylesheet.get(ScssStylesheet.java:172)
  at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:68)
  at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
  at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
  at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:97)
  at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:214)
  at com.vaadin.sass.internal.ScssStylesheet.traverse(ScssStylesheet.java:298)
  at com.vaadin.sass.internal.ScssStylesheet.compile(ScssStylesheet.java:267)
  at com.vaadin.sass.SassCompiler.main(SassCompiler.java:101)
  

• .#{$fa-css-prefix}-2x (see lines 10-13)

  Apr 04, 2016 3:49:57 PM com.vaadin.sass.internal.handler.SCSSErrorHandler log
  SEVERE: Error when parsing file 
  /PATH/fa/_larger.scss on line 10, column 19
  Apr 04, 2016 3:49:57 PM com.vaadin.sass.internal.handler.SCSSErrorHandler log
  SEVERE: encountered "2x". Was expecting one of: "-" "," "[" "%" "." "and" "or" "not" ":" "#{" "through" "in" "{" <IDENT> <HASH> 
  org.w3c.css.sac.CSSParseException: encountered "2x". Was expecting one of: "-" "," "[" "%" "." "and" "or" "not" ":" "#{" "through" "in" "{" <IDENT> <HASH> 
  at com.vaadin.sass.internal.parser.Parser.reportError(Parser.java:418)
  at com.vaadin.sass.internal.parser.Parser.styleRule(Parser.java:2203)
  at com.vaadin.sass.internal.parser.Parser.topLevelDeclaration(Parser.java:591)
  at com.vaadin.sass.internal.parser.Parser.parserUnit(Parser.java:487)
  at com.vaadin.sass.internal.parser.Parser.parseStyleSheet(Parser.java:122)
  at com.vaadin.sass.internal.ScssStylesheet.get(ScssStylesheet.java:172)
  at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:68)
  at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
  at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
  at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:97)
  at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
  at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:214)
  at com.vaadin.sass.internal.ScssStylesheet.traverse(ScssStylesheet.java:298)
  at com.vaadin.sass.internal.ScssStylesheet.compile(ScssStylesheet.java:267)
  at com.vaadin.sass.SassCompiler.main(SassCompiler.java:101)
  

• .#{$fa-css-prefix}-3x (error as per above)

• .#{$fa-css-prefix}-4x (error as per above)

• .#{$fa-css-prefix}-5x (error as per above)

It should be noted that these class token values appear to comply with the W3C CSS Class Selector definition and HTML class attribute. This Sass does compile correctly using LibSass.