diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index b4a6c0637..e2d93476d 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -299,6 +299,7 @@ int mpi_read_string( mpi *X, int radix, const char *s );
  */
 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 @@ int mpi_read_file( mpi *X, int radix, FILE *fin );
  * \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