Plotting without values, using syms

I need to plot theta(t) vs V(t), but have no values for either.

clear all 
close all
 
syms theta t ts tau V0
theta(t)=dsolve('Dtheta=1-theta-t/(ts/tau)')
 
V(t)=V0*(1-t/ts);
 
figure (1)
plot(V(t),theta(t),'o')

I'm getting the error: Non-numeric data is not supported in 'Line'

Is there a way around this?

[Star Strider]

You need values for all the constants as well, for example ‘ts’, ‘tau’, and the initial conditions.