Compiler error with libpq++

Hi;

I am using libpq++ and Standard Template Library to write some programs for
postgres6.5.  I am trying to use map template to store tuples retrieved from
postresql database.  I defined map<string, vector<string> > in my program:        typedef map<string, vector<string> >
SQL_Map;
If I did not include <libpq++.H> and did not use link option from the make
file used in libpq++, also I did not use any class from libpq++.H.
Everything is fine, I can compile the class fine.  I simply use "g++ -c
myclass.cpp" and generate myclass.o file.  However if I include <libpq++.H>
and use the makefile come with libpq++ compiling option, I got an error as:
"/usr/ccs/bin/as: "/var/tmp/cc00POXR.s", line 3512: error: can't compute
value of an expression involving an external symbol"

I believe it has something to do with using string as the key for the map
and using libpq++.H at the same time, because 
map<int, vector<string> > is fine, but I don't know what is the problem.
Could anyone help me out?  Your help will be greatly appreciated.

Sincerely

Wenjin Zheng, Ph.D.
Bioinformatic Analyst
Large Scale Biology, Corp.
Vacaville, CA 95688
wenjin.zheng@lsbc.com