diff --git a/crystal.cpp b/crystal.cpp
index da2f5e2f..08df0280 100644
--- a/crystal.cpp
+++ b/crystal.cpp
@@ -691,9 +691,11 @@ struct hrmap_crystal : hrmap_standard {
 
   virtual transmatrix relative_matrix(cell *h2, cell *h1, const hyperpoint& hint) override { 
     if(!crystal3()) return hrmap_standard::relative_matrix(h2, h1, hint);
+    if(h2 == h1) return Id;
     for(int i=0; i<S7; i++) if(h2 == h1->move(i)) return adj(h1->master, i);
     if(gmatrix0.count(h2) && gmatrix0.count(h1))
       return inverse(gmatrix0[h1]) * gmatrix0[h2];
+    println(hlog, "unknown relmatrix, distance = ", celldistance(h1, h2));
     return xpush(999);
     }