diff --git a/include/polarssl/debug.h b/include/polarssl/debug.h
index c6f7406..e84f4b8 100644
--- a/include/polarssl/debug.h
+++ b/include/polarssl/debug.h
@@ -72,7 +72,7 @@
 
 void debug_print_buf( const ssl_context *ssl, int level,
                       const char *file, int line, const char *text,
-                      unsigned char *buf, int len );
+                      unsigned char *buf, size_t len );
 
 void debug_print_mpi( const ssl_context *ssl, int level,
                       const char *file, int line,