Unexpected Error in fmincon - Unrecognized function or variable 'issystem'

I'm getting the following error while running fconmin -

Unrecognized function or variable 'issystem'.
Error in optim.problemdef.OptimizationProblem/solve

Could someone help me figure out why this error is appearing?

[Rik]

What function are you calling? Can you write a short piece of code that will reproduce this error?

[Neilabh Banzal]

I'm trying, but haven't been able to reproduce it on a simpler example. However, on removing 'OutputSize' argument from fcn2optimexpr, I get the following error -

Error using fcn2optimexpr (line 103)
Function evaluation failed while attempting to determine output size. See error cause below. The function might
contain an error, or might not be well-defined at the automatically-chosen point. To specify output size without
function evaluation, use 'OutputSize'.
Error in main (line 65)
PTmincons  = fcn2optimexpr(@(Q0, P0, L) min(eig(pn(Q0, P0, L, Const))), Q0, P0, L) >= 0;
Caused by:
    Error using feval
    Undefined function 'issystem' for input arguments of type 'double'. 

[Neilabh Banzal]

So, issystem is a part of the Robust Control Toolbox, which is not installed on my system. However, earlier, fmincon never raised this error.

[Rik]

Without code to reproduce this error, it is difficult to suggest a solution.