diff --git a/library/asn1write.c b/library/asn1write.c
index 7803867..a5a35f6 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -99,7 +99,7 @@
 }
 
 #if defined(POLARSSL_BIGNUM_C)
-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 )
 {
     int ret;
     size_t len = 0;