diff --git a/b2/subex.c b/b2/subex.c
index 77b8cbd..b29ae6c 100644
--- a/b2/subex.c
+++ b/b2/subex.c
@@ -92,7 +92,7 @@ static char *compose(const struct chunk *c,
     const char *s, const regmatch_t *match, const char *units)
 {
 	char *res = stralloc("");
-	int res_len = 0, len;
+	int res_len = 0;
 	const char *val;
 	char *tmp, *tmp2;
 	int n;
@@ -118,10 +118,8 @@ static char *compose(const struct chunk *c,
 			}
 			if (match[n].rm_so == -1)
 				break;
-			len = match[n].rm_eo-match[n].rm_so;
-			tmp = alloc_size(len);
-			memcpy(tmp, s+match[n].rm_so, len);
-			tmp[len] = 0;
+			tmp = stralloc_n(s+match[n].rm_so,
+			    match[n].rm_eo-match[n].rm_so);
 			tmp2 = canonicalize(tmp, units ? units[n-1] : 0);
 			append(&res, &res_len, tmp2);
 			free(tmp);