Question: Error, (in plot) unexpected options

Dear all,

I'm trying to plot dispersion curves in Maple but I'm having some trouble. The code is attached as a file also.

f1  is my main function and I want to plot Vx with regard to f as all the rest are known variables, but I'm not able to do so.

I hope that someone can tell me why do I have this error.

Thanks in advance!

restart;
f1 := (C33*Rp*kzp+C13*kx)*(Rm*kx+kzm)*sin(kzp*h)*cos(kzm*h)-(C33*Rm*kzm-C13*kx)*(Rp*kx+kzp)*sin(kzm*h)*cos(kzp*h)=0;
Rp := (-C11*kx^2-C55*kzp^2+omega^2*rho)/((C55+C13)*kx*kzp);
Rm := (-C11*kx^2-C55*kzm^2+omega^2*rho)/((C55+C13)*kx*kzm);
kzp := sqrt(((-M+sqrt(M^2-4*N))*(1/2))*kx^2);
kzm := sqrt(((-M-sqrt(M^2-4*N))*(1/2))*kx^2);
M := (C11*C33/rho^2-2*C55*C13/rho^2+C13^2-omega^2*(C33+C55)/(rho*kx^2))*rho^2/(C33*C55);
N := (omega^2/kx^2-C11/rho)*(omega^2/kx^2-C55/rho)*rho^2/(C33*C55);
C11 := 0.435e10;
C13 := 0.259e10;
C55 := 0.112e10;
C33 := 0.108e11;
rho := 923;
h := 0.7e-2*(1/2);
kx := omega/Vx;
omega := 2*Pi*f;
f1;
plot(f1, f = 10 .. 0.100e6);
Error, (in plot) unexpected options: [(.4633081900*(-0.1717311166e12*f^2/Vx^2-0.2210791386e11*(...

Asim_dispers.mw