Match Errors when a Parser is Optional

[RationalFragile]

Hi, I'm trying to make a very simple example like this:

var value = new NumberParser().WithName("value");
var op = Terminals.Set('+', '-', '*', '/').WithName("op");
var expr = new SequenceParser().WithName("expr");
var opexpr = (op & expr).WithName("opexpr");
expr.Add(value, opexpr.Optional());
var g = new Grammar(expr);
var m = g.Match("1+3*2+4");

And the matcher has m.ErrorMessage

Index=7, Line=1, Context="1+3*2+4>>>"
Expected:
Char: '+','-','*','/'
(Char: '+','-','*','/', (value, Optional: opexpr))

I don't understand why the error happens given than "opexpr" is optional. I tried printing the parsers just to double check, and opexpr is Optional.

Grammar: expr
    expr
        value
        Optional: opexpr
            opexpr
                Char: '+','-','*','/'
                expr

So why does it output an error when "opexpr" fails given that its parent parser is optional? Sorry if I'm doing something obviously wrong.

[cwensley]

Your grammar is fine, the ErrorMessage will be populated whether it is successful or not (it tells you what it expects next), so be sure to check m.Success first.