std.format and floating point issue

[codephantom via Digitalmars-d-learn]

Can anyone help me to understand why the format in the second 
writeln below, does not format the output with commas?

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void main()
{
    import std.stdio, std.format;

    writeln( format("%,.1f", 84543432.951172) ); // 84,543,433.0
    writeln( format("%,.0f", 84543432.951172) ); // 84543433

}

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