diff --git a/library/common.h b/library/common.h
index c175064..2cc44fb 100644
--- a/library/common.h
+++ b/library/common.h
@@ -86,7 +86,7 @@
  * \param   b Pointer to input (buffer of at least \p n bytes)
  * \param   n Number of bytes to process.
  */
-inline void mbedtls_xor( unsigned char *r, unsigned char const *a, unsigned char const *b, size_t n )
+inline void mbedtls_xor( unsigned char *r, const unsigned char *a, const unsigned char *b, size_t n )
 {
     size_t i;
     for ( i = 0; ( i + 4 ) <= n; i += 4 )