diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 154a6ba..dc30206 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -83,7 +83,7 @@
  *
  * \return          the length written or a negative error code
  */
-int asn1_write_mpi( unsigned char **p, unsigned char *start, mpi *X );
+int asn1_write_mpi( unsigned char **p, unsigned char *start, const mpi *X );
 #endif /* POLARSSL_BIGNUM_C */
 
 /**