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-rw-r--r--string.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/string.c b/string.c
index 4197a174fb..450a5590e6 100644
--- a/string.c
+++ b/string.c
@@ -2054,7 +2054,8 @@ rb_str_change_terminator_length(VALUE str, const int oldtermlen, const int terml
long capa = str_capacity(str, oldtermlen);
long len = RSTRING_LEN(str);
- if (capa < len + termlen - oldtermlen) {
+ assert(capa >= len);
+ if (capa - len < termlen - oldtermlen) {
rb_check_lockedtmp(str);
str_make_independent_expand(str, len, 0L, termlen);
}