--- mpmod.c.orig	2006-03-14 16:37:52.000000000 +0100
+++ mpmod.c	2006-03-14 16:40:51.000000000 +0100
@@ -78,7 +78,13 @@
   mpz_mod (w, u, n); /* 2^(2lo) mod n = -/+2^(2lo-l) if m*n = 2^l+/-1 */
   if (mpz_cmp_ui (w, 1) == 0) /* if 2^(2lo) mod n = 1, then n divides 2^lo+1, 
 				 since n has lo+1 bits. */
-    return lo;
+    {
+      if (mpz_scan1 (n, 1) != lo)
+        lo = 0;
+      mpz_clear (w);
+      mpz_clear (u);
+      return lo;
+    }
   k = mpz_sizeinbase (w, 2) - 1;
   /* if w = 2^k then n divides 2^(2*lo-k)-1 */
   mpz_set_ui (u, 1);