Odd error message

Earlier today I created a function which does some database lookups
based on the input parameters.

I ran a query using it and thought it might need some performance
enhancements, so I created an index on two of the columns used within the
function to see if that would make it run faster.  It didn't, so I dropped
that index and created a different one, adding a third column to the index.

I then got the following error message:

WARNING:  Error occurred while executing PL/pgSQL function uscf_status_on
WARNING:  line 8 at select into variables
ERROR:  Relation 3912941 does not exist

Recreating the function got rid of the error message, though I didn't
get the performance improvement I was looking for.

When I delete that new index (which didn't help the performance, either),
the error message returns.  Recreating the function gets rid of it again.

What is going wrong to produce that error message?  (Running 7.3.3)

--
Mike Nolan