diff --git a/include/polarssl/asn1write.h b/include/polarssl/asn1write.h
index 6bac6c4..f7efb20 100644
--- a/include/polarssl/asn1write.h
+++ b/include/polarssl/asn1write.h
@@ -33,7 +33,9 @@
 
 int asn1_write_len( unsigned char **p, unsigned char *start, size_t len );
 int asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag );
+#if defined(POLARSSL_BIGNUM_C)
 int asn1_write_mpi( unsigned char **p, unsigned char *start, mpi *X );
+#endif
 int asn1_write_null( unsigned char **p, unsigned char *start );
 int asn1_write_oid( unsigned char **p, unsigned char *start, const char *oid );
 int asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start, const char *algorithm_oid );