diff --git a/library/bignum.c b/library/bignum.c
index 0cb9505..5c0d622 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -173,7 +173,7 @@
 /*
  * Get a specific bit
  */
-int mpi_get_bit( mpi *X, size_t pos )
+int mpi_get_bit( const mpi *X, size_t pos )
 {
     if( X->n * biL <= pos )
         return( 0 );