diff --git a/ChangeLog b/ChangeLog
index e13bf3e03..3a63c4420 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,6 +12,8 @@ Bug fixes
      certificate in the chain in ssl_write_certificate() and
 	 ssl_write_certificate_request() (Found by fatbob)
    * Added small fixes for compiler warnings on a Mac
+   * Fixed algorithmic bug in mpi_is_prime() (Found by
+     Smbat Tonoyan)
 
 = Version 0.12.1 released on 2009-10-04
 Changes
diff --git a/library/bignum.c b/library/bignum.c
index 357dd4601..fa9f1f141 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1720,8 +1720,8 @@ int mpi_is_prime( mpi *X, int (*f_rng)(void *), void *p_rng )
      * W = |X| - 1
      * R = W >> lsb( W )
      */
-    s = mpi_lsb( &W );
     MPI_CHK( mpi_sub_int( &W, X, 1 ) );
+    s = mpi_lsb( &W );
     MPI_CHK( mpi_copy( &R, &W ) );
     MPI_CHK( mpi_shift_r( &R, s ) );