Question: Invalid assignment error?

I have these three functions:

Phi := proc (z) options operator, arrow; z^(-(1/2)*alpha)*(BesselJ(alpha, 2*sqrt(lambda*alpha*z))-BesselY(alpha, 2*sqrt(lambda*alpha*z))*BesselJ(alpha, 2*sqrt(lambda*alpha*a))/BesselY(alpha, 2*sqrt(lambda*alpha*a))) end proc

w := proc (z) options operator, arrow; alpha*z^alpha end proc

f := proc (z) options operator, arrow; c[0]-c[r](z/a)^(-alpha) end proc

And I have made the assumption that, 0<a<1. I then try and evaluate these intergrals,

I1 := int(f(z)*Phi(z)*w(z), z = a .. 1)=int((c[0]-c[r](z/a)^(-alpha))*z^(-(1/2)*alpha)*(BesselJ(alpha, 2*sqrt(lambda*alpha*z))-BesselY(alpha, 2*sqrt(lambda*alpha*z))*BesselJ(alpha, 2*sqrt(lambda*alpha*a))/BesselY(alpha, 2*sqrt(lambda*alpha*a)))*alpha*z^alpha, z = a .. 1)

I2 := int(Phi(z)^2*w(z), z = a .. 1)

which produces no errors...However on dividing one by the other...

A[N] := I1/I2

I get a "Error, invalid assignment" message.... I don't think I'm dividing by zero anywhere... Is there something glaringly obvious that I've done wrong, or some way to get around this problem?...

Thanks k