How to Set Plateform Specific Padding Value?

I want to set platform specific padding value, so I made setter like this:

    <Setter Property="Padding">
        <Setter.Value>
            <OnPlatform x:TypeArguments="x:Double" 
                Android="10, 0, 10, 0"
                WinPhone="10, 0, 10, 0"
                iOS="10, 20, 10, 0"
            />
        </Setter.Value>
    </Setter>

But I got exception "Input string was not in a correct format".

Please let me know about this reason.
Thanks