Question: Problem with "pdsolve" function

Hi all. I'm using pdsolve to find linearly independent solutions for two partial differentioal equations. I can solve them seperatly by "pdsolve", but when I try to solve them at the same time to get answers that fit in both of the eauations, I get an error message. I need to be able to solve them as a set of equations, as I have bigger examples. Here are the equations and outputs: 

PDE[1] := diff(f(theta[I], theta[A], theta[B], theta[AB]), theta[AB]);

PDE[2] := -(diff(f(theta[I], theta[A], theta[B], theta[AB]), theta[I]))+diff(f(theta[I], theta[A], theta[B], theta[AB]), theta[A])+diff(f(theta[I], theta[A], theta[B], theta[AB]), theta[B]);

pdsolve(PDE[1] = 0)  ----->  f(theta[I], theta[A], theta[B], theta[AB]) = _F1(theta[I], theta[A], theta[B])

pdsolve(PDE[2] = 0) ----->  f(theta[I], theta[A], theta[B], theta[AB]) = _F1(theta[I]+theta[A], theta[I]+theta[B], theta[AB])

pdsolve({seq(PDE[i] = 0, i = 1 .. 2)}) ---->  Error, (in PDEtools:-casesplit) too many levels of recursion

Why doesn't it produce the answer, although I know that it must be:

f(theta[I], theta[A], theta[B], theta[AB])=_F1(theta[I]+theta[A], theta[I]+theta[B])