stylus: ParseError: expected "indent", got "outdent" (using hashes)

The following Stylus fails when not using a colon after background-color but works with one:

Failure

$Colors = {'Red': #ff0000}

body
    background-color $Colors.Red

> stylus < test.styl 

/usr/local/lib/node_modules/stylus/bin/stylus:494
      if (err) throw err;
                     ^
ParseError: stdin:4
   1| $Colors = {'Red': #ff0000}
   2| 
   3| body
 > 4|     background-color $Colors.Red
   5| 

expected "indent", got "outdent"

    at Parser.error (/usr/local/lib/node_modules/stylus/lib/parser.js:179:11)
    at Parser.expect (/usr/local/lib/node_modules/stylus/lib/parser.js:207:12)
    at Parser.block (/usr/local/lib/node_modules/stylus/lib/parser.js:688:12)
    at Parser.selector (/usr/local/lib/node_modules/stylus/lib/parser.js:1224:24)
    at Parser.property (/usr/local/lib/node_modules/stylus/lib/parser.js:1175:47)
    at Parser.ident (/usr/local/lib/node_modules/stylus/lib/parser.js:1130:25)
    at Parser.stmt (/usr/local/lib/node_modules/stylus/lib/parser.js:632:26)
    at Parser.statement (/usr/local/lib/node_modules/stylus/lib/parser.js:540:21)
    at Parser.block (/usr/local/lib/node_modules/stylus/lib/parser.js:700:21)
    at Parser.selector (/usr/local/lib/node_modules/stylus/lib/parser.js:1224:24)

Success

$Colors = {'Red': #ff0000}

body
    background-color: $Colors.Red

> stylus < test.styl 
body {
  background-color: #f00;
}