Identifying parameters in state space model misinterprets I/O count

Hi,

I'm trying to take some collected data, and fit it into a structured state space model, fixing some of the parameters and allowing the others to be estimated by MATLAB. I'm using for data vectors to do this. Three 610x1 doubles for output, and One 610x1 double for input.

% Setup Parameter Matrices
V = 1.00;
L1 = 0.255;
L2 = 0.845;
A = [0,  V, (V*L1)/(L1+L2);
   0,  0,  V/(L1+L2);
   0,  0,  0];
B = [ 0;
    0;
    1 ];
C = [ 1, 1, 1];
D = 0;
K = [ 0; 0; 0 ];

% Some elements are fixed elements
mp = idss(A,B,C,D,K);
mp.Structure.A.Free = (A~=0);
mp.Structure.B.Free = 0;
mp.Structure.C.Free = 0;
mp.Structure.K.Free = 0;

% Create iddata, is my formatting here where I'm causing an error?
robot = iddata([XTE_d PSI_d Delta_d],[u_d],0.1);
robot.InputName = 'Steer Cmd';
robot.OutputName = {'XTE' 'PSI' 'Delta'};
robot.InputUnit = {'deg/S'};
robot.OutputUnit = {'Meters' 'Deg' 'Deg'}
robot.Tstart = 0;
robot.TimeUnit = 's';

mp = ssest(robot,mp)

Error using ssest (line 132)
The number of inputs and outputs of the model must match that of the data.

So by running it:

Error in readAndPlot (line 83)
mp = ssest(robot,mp)

As far as I can tell my iddata object is correct.

robot =

Time domain data set with 610 samples.
Sample time: 0.1 seconds               

Outputs         Unit (if specified)    
 XTE             Meters              
 PSI             Deg                 
 Delta           Deg                 

Inputs          Unit (if specified)    
 Steer Cmd       deg/S

3 outputs and 1 input - yeah?

I found a similar question here: Question

And an example I'm following is here: Estimate Structured Discrete-Time State-Space Models

What's happening? Running

mp = ssest(robot(1:305),3,'Ts',0.1);

Seems to work just fine. Why can't I pass in an idss object successfully?