diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index b4a6c06..e2d9347 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -299,6 +299,7 @@
  */
 int mpi_write_string( const mpi *X, int radix, char *s, size_t *slen );
 
+#if defined(POLARSSL_FS_IO)
 /**
  * \brief          Read X from an opened file
  *
@@ -325,6 +326,7 @@
  * \note           Set fout == NULL to print X on the console.
  */
 int mpi_write_file( const char *p, const mpi *X, int radix, FILE *fout );
+#endif /* POLARSSL_FS_IO */
 
 /**
  * \brief          Import X from unsigned binary data, big endian