Question: Problems solving pdes - Error, (in pdsolve/indexlex) invalid input

Hi everyone,

I'm having trouble solving a set of pdes and keep encountering the following error:

Error, (in pdsolve/indexlex) invalid input: a string/name list is expected for sort method `string`

I have been trying to find out why I get the error or what the indexlex error is but am stumped... I cannot find anything on the Maplesoft or mapleprimes websites about it.

This is the code I am using which is giving the error:

equations:={

diff(c[1](t,z),t)+16/9*diff(q[1](t,z),t)+7.24+1.74e13*c[1](t,z)^12*diff(c[1](t,z),z)=

300+7.4e14*c[1](t,z)^12*diff(c[1](t,z),z,z),

diff(q[1](t,z),t)=(1.35e10*c[1](t,z)^13*(1-q[1])-q[1](t,z))/(2/3+1.35e14*c[1](t,z)^12*(1-1/100*q[1]))

};

ibc := {c[1](t,0)=1, q[1](0,z)=0};

variables := {q[1](t,z),c[1](t,z)};

sol:=pdsolve(equations, ibc, numeric, variables);

Thanks for any help or suggestions you can provide

Dan