Question: How to Graph Overlaying Plots With No Errors?

I am trying to plot some graphs for a differential equations class. I need to plot two equations (soln1a, soln1b1) in one graph, with the two curves overlayed on each other. I am trying to plot them on a graph named "gr1c". The error I get says "Error, (in plots/multiple) empty plot". Clicking on the error results in a webpage saying that "There is no help page available for this error".

When I plot each equation separately, there are no errors. The graphing command that works for soln1a is 

gr1a:=plot(rhs(soln1a),dom1,color=blue);

and the graphing command that works for soln1b1 is

gr1b1:=plot(rhs(soln1b_1),dom1,color=purple);
 

My code is as follows. I would greatly appreciate any help ASAP to remove the error. Thank you!

with(plots):with(DEtools):

K:=9;
deG:=diff(theta(t),t,t) + mu*diff(theta(t),t)+K*sin(theta(t))= 0;
deL:=diff(theta(t),t,t) + mu*diff(theta(t),t)+K*theta(t)= 0;
Iv:=theta(0)=0.75, D(theta)(0)=2.0;
dom1:=t=0..10;
soln1a:=dsolve({eval(deL,mu=0),Iv});

soln1b_1:=dsolve({eval(deL,mu=1),Iv});

gr1c:=multiple(plot[soln1a,soln1b1],dom1,color=[blue,purple]);