1 files changed, 1 insertions, 1 deletions
diff --git a/string.c b/string.c
index 4bc4ebfd99..514488fd93 100644
--- a/string.c
+++ b/string.c
@@ -2570,7 +2570,7 @@ str_buf_cat(VALUE str, const char *ptr, long len)
     rb_str_modify(str);
     if (len == 0) return 0;
     if (STR_EMBED_P(str)) {
-	capa = RSTRING_EMBED_LEN_MAX + termlen - 1;
+	capa = RSTRING_EMBED_LEN_MAX + 1 - termlen;
 	sptr = RSTRING(str)->as.ary;
 	olen = RSTRING_EMBED_LEN(str);
     }