--- lib/ctblfuns.gi.orig	2018-11-01 16:56:12.000000000 -0600
+++ lib/ctblfuns.gi	2019-02-12 08:43:52.769804253 -0700
@@ -4319,7 +4319,7 @@ InstallGlobalFunction( ReductionToFinite
 
     if k <> 1 then
 
-      primes:= PrimeDivisors( m );
+      primes:= ShallowCopy( PrimeDivisors( m ) );
       sol:= fail;
 
       while not IsEmpty( primes ) do