diff --git a/library/bignum.c b/library/bignum.c
index 5635bd2..eabfbfe 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2033,6 +2033,7 @@
     if( verbose != 0 )
         printf( "passed\n" );
 
+#if defined(POLARSSL_GENPRIME)
     MPI_CHK( mpi_inv_mod( &X, &A, &N ) );
 
     MPI_CHK( mpi_read_string( &U, 16,
@@ -2053,6 +2054,7 @@
 
     if( verbose != 0 )
         printf( "passed\n" );
+#endif
 
     if( verbose != 0 )
         printf( "  MPI test #5 (simple gcd): " );