diff --git a/src/tools/NeighborList.cpp b/src/tools/NeighborList.cpp
index 65f3c9a2219314a9c1817202613117a1a15bceaf..8866378c7909e5ea5e8c33d00fa2700ac506d6b6 100644
--- a/src/tools/NeighborList.cpp
+++ b/src/tools/NeighborList.cpp
@@ -45,7 +45,7 @@ NeighborList::NeighborList(const vector<AtomNumber>& list0, const vector<AtomNum
   if(!do_pair) {
     nallpairs_=nlist0_*nlist1_;
   } else {
-    plumed_assert(nlist0_==nlist1_);
+    plumed_massert(nlist0_==nlist1_,"when using PAIR option, the two groups should have the same number of elements");
     nallpairs_=nlist0_;
   }
   initialize();