Runaway error in section title

Question

I'm using overleaf and getting errors while trying to run

\subsection{Decomposition of circular functions as a product of elemental trigonometric functions \footnote{We take elemental trigonometric functions to mean circular functions having a period of $2 \pi$ (i.e. $\sin x$, $\cos x$).}} 

The error im getting is:

Argument of \@sect has an extra }.

<inserted text> 
                \par 
l.24 ...od of $2 \pi$ (i.e. $\sin x$, $\cos x$).}}

I've run across a `}' that doesn't seem to match anything.
For example, `\def\a#1{...}' and `\a}' would produce
this error. If you simply proceed now, the `\par' that
I've just inserted will cause me to report a runaway
argument that might be the root of the problem. But if
your `}' was spurious, just type `2' and it will go away.
 main.tex, line 24

Runaway argument?

{\normalfont \large \bfseries }{\@xfootnote }\def \reserved@b {\stepcounter \ETC.
! Paragraph ended before \@sect was complete.
<to be read again> 
                   \par 
l.24 ...od of $2 \pi$ (i.e. $\sin x$, $\cos x$).}}

I suspect you've forgotten a `}', causing me to apply this
control sequence to too much text. How can we recover?
My plan is to forget the whole thing and hope for the best.