You probably inserted the TOA field inside the TOC field.  Press Alt+F9
to switch to display of field codes.  For the TOC field, you should see
something like:

{ TOC \o "1-3" \h \z \u }

and my guess is that buried somewhere between the pair of curly braces
for the TOC field are another pair of curly braces for the TOA field.
Just move the TOA field outside of the TOC field braces.