diff --git a/include/string.h b/include/string.h
index 006fdf0..83a1654 100644
--- a/include/string.h
+++ b/include/string.h
@@ -13,7 +13,7 @@ String * string_new(String * string);
 void string_delete(String * string);
 
 /* useful */
-int string_append(String * string, String * append);
+int string_append(String ** string, String * append);
 void string_cut(String * string, unsigned int length);
 int string_length(String * string);