diff --git a/rulegen3.cpp b/rulegen3.cpp
index ff7857ab..28c2f5d2 100644
--- a/rulegen3.cpp
+++ b/rulegen3.cpp
@@ -1386,6 +1386,7 @@ vector<int> representative;
 
 void genhoneycomb(string fname) {
   if(WDIM != 3) throw hr_exception("genhoneycomb not in honeycomb");
+  if(!known()) throw hr_exception("rules not known");
 
   int qc = isize(t_origin);