diff --git a/io.c b/io.c
index 7aba8be12f..ed8bc1ee3b 100644
--- a/io.c
+++ b/io.c
@@ -9017,6 +9017,7 @@ rb_f_backquote(VALUE obj, VALUE str)
     GetOpenFile(port, fptr);
     result = read_all(fptr, remain_size(fptr), Qnil);
     rb_io_close(port);
+    RFILE(port)->fptr = NULL;
     rb_io_fptr_finalize(fptr);
     rb_gc_force_recycle(port); /* also guards from premature GC */