std.algorithm range violation

[maarten van damme via Digitalmars-d-learn]

an anyone explain me what I'm doing wrong here :

[code]
dstring[][dstring] providor_symbol_map;
...

writeln(providor_symbol_map.keys.sort!((x,y)=>providor_symbol_map[x].length>=providor_symbol_map[y].length));
[/code]

output:
core.exception.RangeError at std.algorithm(9429): Range violation
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