Creating function and aggregate - problem

I have tried to create a min(oid) aggregate, but when I use it, I get
the message `ERROR:  fmgr_info: function 108994: cache lookup failed'.

What does this mean, please?

I created it thus:

create function oid4smaller (oid, oid) returns oid as
                '/home/olly/cprogs/oidcompare.so' language 'c';

create aggregate min (basetype = oid, sfunc1 = oid4smaller,
                      stype1 = oid, stype2 = oid);


The C file is compiled and linked thus (for Linux x86):

  $ gcc -o oidcompare.o -c -I/usr/include/postgresql oidcompare.c
  $ gcc -shared -o oidcompare.so oidcompare.o

and it says:
#include <postgresql/postgres.h>

Oid oid4smaller(Oid o1, Oid o2) {
        return (o1 < o2 ? o1 : o2);
}

Oid oid4larger(Oid o1, Oid o2) {
        return (o1 > o2 ? o1 : o2);
}

--
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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     "The LORD is nigh unto all them that call upon him, to
      all that call upon him in truth."
                                   Psalms 145:18