jtraj for underactuated robot

I am trying to calculate joint space trajectory of an underactuated robot using .jtraj function of Peter Corke robotics toolbox. Default inverse kinematics function used in the command is ikine6s. According to documentation,

q = R.jtraj(T1, t2, k, options) , where we can pass ikine command in the options for underacrtuated robot. But when I do so, I get the error message-

w=R.jtraj(T1,t2,10000,R.ikine(T1,T0i2{2}(1:4,4)',m));
%output 
Error using SerialLink/ikine6s (line 57)
Solution only applicable for 6-axis robot
Error in SerialLink>@(varargin)r.ikine6s(varargin{:}) (line 722)
                opt.ikine = @r.ikine6s;
Error in SerialLink/jtraj (line 726)
            q1 = opt.ikine(T1, args{:}); 

where T1 is 4x4 homogeneous matrix, T0i2{2}(1:4,4) is 1x4 matrix and m is the mask matrix.

I get the same error message by using ikcon and ikunc. Is there any work around for this?

Thanks in advance.