diff --git a/lib/grp.gi b/lib/grp.gi
index 2f1c131b96..373ea5bf72 100644
--- a/lib/grp.gi
+++ b/lib/grp.gi
@@ -662,7 +662,9 @@ local new,i,c;
     if Size(new[Length(new)])>Size(c) then
       Add(new,c);
     fi;
-    Add(new,ser[i]);
+    if Size(new[Length(new)])>Size(ser[i]) then
+      Add(new,ser[i]);
+    fi;
     sub:=Intersection(sub,ser[i]);
     i:=i+1;
   od;
@@ -673,6 +675,7 @@ local new,i,c;
     Add(new,ser[i]);
     i:=i+1;
   od;
+  Assert(1,ForAll([1..Length(new)-1],x->Size(new[x])<>Size(new[x+1])));
   return new;
 end);