What does "merge-joinable join conditions" mean ????

I have a FULL JOIN between two simple tables, and am getting the message 
" Query failed: ERROR:  FULL JOIN is only supported with merge-joinable 
join conditions"

I'd be glad to fix this in my query if I knew what the #@!! it meant.  
Using 8.0.4 on FC1;  willing to upgrade (PostgreSQL, not FC1) IF it 
would fix the problem.

-- Dean