diff --git a/src/multiset.h b/src/multiset.h
index 49978ed0a99d14433d38e61f5fe2e8093b043881..d9408e92a1573578f78041f0bbd91ed36d18853d 100644
--- a/src/multiset.h
+++ b/src/multiset.h
@@ -15,7 +15,7 @@ struct multiset
   struct array *ind;
 };
 
-struct multiset *first_multiset;
+extern struct multiset *first_multiset;
 
 #define free_multiset(L) do{ struct multiset *l_=(L); if(!--l_->refs) really_free_multiset(l_); }while(0)