msgpack unresolved question

[Dan]

I got msgpack and was trying out an example (compare_json.d) and 
I get an unresolved error.
Not sure what I'm doing wrong...
https://github.com/msgpack/msgpack-d/blob/master/src/msgpack.d
I think length is built in for associative arrays?

Thanks
Dan

The error and lines of code is:
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/tmp/.rd.../objs/compare_json.o: In function `const(@trusted void 
function(ref 
msgpack.Packer!(std.array.Appender!(ubyte[]).Appender).Packer)) 
msgpack.Value.toMsgpack!(msgpack.Packer!(std.array.Appender!(ubyte[]).Appender).Packer).toMsgpack':
/home/dbdavidson/stage/msgpack-d/src/msgpack.d:2967: undefined 
reference to `@property ulong 
object.AssociativeArray!(msgpack.Value, 
const(msgpack.Value)).AssociativeArray.length()'


        case Type.array:
            packer.beginArray(via.array.length);
            foreach (elem; via.array)
                elem.toMsgpack(packer);
            break;
        case Type.map:
        packer.beginMap(via.map.length);
            foreach (key, value; via.map) {
                key.toMsgpack(packer);
                value.toMsgpack(packer);
            }
            break;
        }

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