diff --git a/include/tlibc/string/str.h b/include/tlibc/string/str.h
index bad3717..e005df5 100755
--- a/include/tlibc/string/str.h
+++ b/include/tlibc/string/str.h
@@ -19,7 +19,7 @@ str s = STR("something");
 printf(FMT_str"\n", str_expand(s));
 */
 #define FMT_str "%.*s"
-#define str_expand(s) s.len, s.data
+#define str_expand(S) (S).len, (S).data
 
 /// creates str from a string literal
 #define STR(LITERAL) str_construct(LITERAL, ARRAY_LEN(LITERAL) - 1, true)