diff --git a/include/mbedtls/debug.h b/include/mbedtls/debug.h
index fcf1490..8a4da1c 100644
--- a/include/mbedtls/debug.h
+++ b/include/mbedtls/debug.h
@@ -124,7 +124,7 @@
 
 void debug_print_buf( const ssl_context *ssl, int level,
                       const char *file, int line, const char *text,
-                      unsigned char *buf, size_t len );
+                      const unsigned char *buf, size_t len );
 
 #if defined(POLARSSL_BIGNUM_C)
 void debug_print_mpi( const ssl_context *ssl, int level,