diff --git a/string.c b/string.c
index 77f360b8d2..7f0468ff19 100644
--- a/string.c
+++ b/string.c
@@ -1346,7 +1346,7 @@ str_new_empty(VALUE str)
     return v;
 }
 
-#define STR_BUF_MIN_SIZE 127
+#define STR_BUF_MIN_SIZE 63
 STATIC_ASSERT(STR_BUF_MIN_SIZE, STR_BUF_MIN_SIZE > RSTRING_EMBED_LEN_MAX);
 
 VALUE