[CppAD] yq = f.Forward(q, xq): a zero order Taylor coefficient is nan. What usually causes it?

[Hank xu]

Hi,

I'm a new user of CppAD library and am trying to use it with Ipopt. I
implemented my formulation following the format from this example
<https://www.coin-or.org/CppAD/Doc/ipopt_solve_get_started.cpp.htm>, and
now is stuck with this error;

 yq = f.Forward(q, xq): a zero order Taylor coefficient is nan

After both searching online and looking through source code, I could not
figure out the root cause of this error. I'm not too familiar with the
automatic differentiation field and would love to get some help on how to
tackle this. Any help would be greatly appreciated!

Best,
Haoyun
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