Question: How to fix empty 3d plots?

Hi, I'm trying to plot the following functions in 3 dimensions using the following:

plot3d(kappa*sech(kappa*(-k^2*t+x)), x = -10 .. 10, t = -10 .. 10)

plot3d((2*(-K^2+k^2))*(K*cosh(k*(-k^2*t+x))+k*cosh(K*(-K^2*t+x)))/((K+k)^2*cosh(k*(-k^2*t+x)-K*(-K^2*t+x))+(-K+k)^2*cosh(k*(-k^2*t+x)+K*(-K^2*t+x))+4*K*k), x = -10 .. 10, t = -10 .. 10)

plot3d(2*(diff(arctan(lambda*sin(mu*((-3*lambda^2+mu^2)*t+x))*sech(lambda*((-lambda^2+3*mu^2)*t+x))/mu), x)), x = -10 .. 10, t = -10 .. 10);

Each produces an empty plot with a message similar to the following:
Warning, expecting only range variables [x, t] in expression 2*(lambda*cos(mu*((-3*lambda^2+mu^2)*t+x))*sech(lambda*((-lambda^2+3*mu^2)*t+x))-lambda^2/mu*sin(mu*((-3*lambda^2+mu^2)*t+x))*sech(lambda*((-lambda^2+3*mu^2)*t+x))*tanh(lambda*((-lambda^2+3*mu^2)*t+x)))/(lambda^2/mu^2*sin(mu*((-3*lambda^2+mu^2)*t+x))^2*sech(lambda*((-lambda^2+3*mu^2)*t+x))^2+1) to be plotted but found names [lambda, mu]

in each case kappa, k, K, lambda, mu are real constants 

Thanks for any help with this