diff --git a/ChangeLog b/ChangeLog
index 4ae771b..ecfb0c6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@
   * Support for DTLS 1.0 and 1.2 (RFC 6347).
 
 API Changes
+   * Some constness fixes
    * Signature of mpi_mul_mpi() changed to make the last argument unsigned
    * Remove the PBKDF2 module (use PKCS5).
    * Remove POLARSSL_ERROR_STRERROR_BC (use mbedtls_strerror()).
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 44248f3..171d42a 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -715,7 +715,7 @@
  *                 POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed,
  *                 POLARSSL_ERR_MPI_NOT_ACCEPTABLE if X is not prime
  */
-int mpi_is_prime( mpi *X,
+int mpi_is_prime( const mpi *X,
                   int (*f_rng)(void *, unsigned char *, size_t),
                   void *p_rng );
 
diff --git a/library/bignum.c b/library/bignum.c
index 2d04eb4..8a751a3 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2041,7 +2041,7 @@
 /*
  * Pseudo-primality test: small factors, then Miller-Rabin
  */
-int mpi_is_prime( mpi *X,
+int mpi_is_prime( const mpi *X,
                   int (*f_rng)(void *, unsigned char *, size_t),
                   void *p_rng )
 {