Mixing periodic and non-periodic boundary conditions in a simple PDE system

Questions
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1

Hi dear community,

I am trying to solve a simple PDE system in the form of:

systemofequations.JPG

where Na, ni are functions of [x,t] and va, vi, beta are constant coefficients.
I have no problem solving the system with IC and non-periodic BC:

Na[x,0]=Na0; Na[0,t]=Na0; 

ni[x,0]=ni0; ni[0,t]=ni0

However, when trying to implement a periodic boundary condition for ni:

ni[0,t]=ni[L,t]

and keeping the BC for Na the left boundary non-periodic: Na[0,t]=Na0 Mathematica states that such a combination is not allowed:

mixingboundarycondtions.png

Has anybody encountered such a problem and has found a workaround for mixing periodic and non-periodic boundary conditions?