Figure caption error

[alirezaei]

I copied my paper tex file in my dissertation with ucla style.
the first time I run the typeset, it goes without any error, then when I run it for the second time to add reference numbers it gives me the following error.

! Missing $ inserted.

$
l.19 ...} \leq 1$.\relax }}{13}{figure.caption.12}

This error is because of the caption of one of the figures, the caption is:

\caption{After the intersection point $a$, the trajectory can stay on the hyperplane $\mathcal{H}{uc{ij}}$ or leave it with the slope $\underline{s}{ij} \leq s{ij} \leq 1$.}

I used this caption in my paper with aiaa style without any problem but it does not work with ucla style.

Any help will be greatly appreciated
Thanks