I have a set of partial differential equations and want to find its conserved currents using PDE pakage. 

I define second order partial differential equations as q1,q2,q3,q4 and a set of them: PDE3:={q1,q2,q3,q4} and use the command J[alpha] := ConservedCurrents(PDE3) to calculate its conserved currents but I encounter this error which made me crazy!

Error, (in PDEtools:-DeterminingPDE) invalid input: PartialDerivatives expects value for keyword parameter maxdifforder to be of type nonnegint, but received -1

my equations are like

((1/2)*(diff(R(theta, psi), theta))*(diff(f(theta, psi), theta))+(3/8)*(diff(R(theta, psi), theta))^2+(1/8)*(diff(f(theta, psi), theta))^2+(1/4)*(diff(f(theta, psi), theta, theta))+(1/4)*(diff(R(theta, psi), theta, theta)))*exp(f(theta, psi))+diff(R(theta, psi), psi, psi)-(1/2)*(diff(R(theta, psi), psi))*(diff(f(theta, psi), psi))+(1/2)*(diff(chi(theta, psi), psi))^2 = 0

I don't know what's wrong, can you help me?