NPE when on missing expression for a declaration

[silentmatt]

p {
    margin
}

Syntax Error: line 3 col 1: ":" expected
Exception in thread "main" java.lang.NullPointerException
    at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:191)
    at com.google.common.collect.SingletonImmutableList.<init>(SingletonImmutableList.java:40)
    at com.google.common.collect.ImmutableList.copyFromCollection(ImmutableList.java:290)
    at com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:253)
    at com.silentmatt.dss.Expression$Builder.build(Expression.java:56)
    at com.silentmatt.dss.parser.Parser.expr(Parser.java:515)
    at com.silentmatt.dss.parser.Parser.multideclaration(Parser.java:850)
    at com.silentmatt.dss.parser.Parser.declarationBlock(Parser.java:774)
    at com.silentmatt.dss.parser.Parser.ruleset(Parser.java:195)
    at com.silentmatt.dss.parser.Parser.rule(Parser.java:173)
    at com.silentmatt.dss.parser.Parser.CSS3(Parser.java:156)
    at com.silentmatt.dss.parser.Parser.Parse(Parser.java:1611)
    at com.silentmatt.dss.parser.DSSParser.parse(DSSParser.java:45)
    at com.silentmatt.dss.DSSDocument.parse(DSSDocument.java:87)
    at com.silentmatt.dss.DSSDocument.parse(DSSDocument.java:72)
    at com.silentmatt.dss.DSSDocument.parse(DSSDocument.java:57)
    at com.silentmatt.dss.Main.processFile(Main.java:30)
    at com.silentmatt.dss.Main.main(Main.java:290)