I can't type commas in an numeric entry binded to a decimal value

This is really really strange.

I have an entry binded to a ´decimal´ value with a default value (which comes from server):

    <Entry Text="{Binding AmountToPay}" Keyboard="Numeric" />

And I see the value correctly, but if I remove the value and I try to type a different value, I can't introduce a comma. For example, I can't introduce 12,25, but I can introduce 12.

Do somebody know what is happening?