diff --git a/string.c b/string.c
index a31d53c4c33e8b..938edd9cdf336a 100644
--- a/string.c
+++ b/string.c
@@ -3094,7 +3094,7 @@ str_buf_cat(VALUE str, const char *ptr, long len)
     return str;
 }
 
-#define str_buf_cat2(str, ptr) str_buf_cat((str), (ptr), strlen(ptr))
+#define str_buf_cat2(str, ptr) str_buf_cat((str), (ptr), rb_strlen_lit(ptr))
 
 VALUE
 rb_str_cat(VALUE str, const char *ptr, long len)