Unrecognized function 'issystem' in feval

I'm getting the following error while running fconmin -

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'. 

From what I understand, issystem is a part of the Robust Control Toolbox, which is not installed on my system. However, earlier, fmincon never raised this error. I'm extremely confused as to why this error is appearing.

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