diff --git a/io.c b/io.c
index 1a22d34c50..442bf52e40 100644
--- a/io.c
+++ b/io.c
@@ -3001,6 +3001,7 @@ io_write_nonblock(VALUE io, VALUE str, VALUE ex)
 
     rb_io_set_nonblock(fptr);
     n = write(fptr->fd, RSTRING_PTR(str), RSTRING_LEN(str));
+    RB_GC_GUARD(str);
 
     if (n < 0) {
 	int e = errno;