Verify that calling .add() outside vcl_init invokes VCL failure.

src/tests/match.vtc

@@ -787,3 +787,31 @@ varnish v1 -errvcl {vmod selector failure: t.add(): "foo" added more than once}
 		t.add("foo");
 	}
 }
+
+varnish v1 -vcl {
+	import ${vmod_selector};
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		new t = selector.set();
+		t.add("foo");
+	}
+
+	sub vcl_recv {
+		t.add("bar");
+		return (synth(200));
+	}
+}
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 503
+	expect resp.reason == "VCL failed"
+} -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error {^vmod selector failure: t\.add\(\) may only be called in vcl_init$}
+	expect * = End
+} -run