Question about Races with casting...

[WeaselGuy]

So, my question is this: if I take a race that has casting as per a caster class (for this example, let's say a Rakshasa, which has casting equal to a 7th level sorcerer, and taking a level of sorcerer increases stacks with it's racial casting), and I want to take a prestige class that adds +1 of an existing class level, do I need to take a level of sorcerer before going into that prestige class in order to increase the casting?

To clarify, in case that's confusing...

Rakshasa, casts as a 7th level Sorcerer. Wants to take a level of Abjurant Champion. Do I need an actual level of Sorcerer in order to get the casting progression from AbJ, or will it increase the racial casting?

[snailgosh]

I'm going out on a limb and sai it's RAI that PrC's that advance spellcasting also apply to 'monster class' levels with innate spellcasting.