diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 0b51e81..941b7be 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -1053,6 +1053,8 @@ zstream_run_func(struct zstream_run_args *args)
     while (!args->interrupt) {
         n = z->stream.avail_out;
 
+        rb_str_locktmp(z->buf);
+
 #ifndef RB_NOGVL_UBF_ASYNC_SAFE
         err = (int)(VALUE)rb_thread_call_without_gvl(zstream_run_once, (void *)args, zstream_unblock_func, (void *)args);
 #else
@@ -1060,6 +1062,7 @@ zstream_run_func(struct zstream_run_args *args)
 #endif
 
         rb_str_set_len(z->buf, ZSTREAM_BUF_FILLED(z) + (n - z->stream.avail_out));
+        rb_str_unlocktmp(z->buf);
 
         if (err == Z_STREAM_END) {
             z->flags &= ~ZSTREAM_FLAG_IN_STREAM;