diff --git a/internal/smallwebwaf/healthcheck_test.go b/internal/smallwebwaf/healthcheck_test.go
index 3089f22..a316c6c 100644
--- a/internal/smallwebwaf/healthcheck_test.go
+++ b/internal/smallwebwaf/healthcheck_test.go
@@ -60,8 +60,13 @@ func TestHealthCheck(t *testing.T) {
 		}, 0, "")
 	}
 
-	app.Close()
-	wantHealthCheck(t, env, 1, "unhealthy: connect to the app: ")
+	// Another test can take a closed server's port at once, so the app's
+	// address here is one that nothing can listen on: TCP cannot connect
+	// to a multicast address such as 224.0.0.1.
+	wantHealthCheck(t, map[string]string{
+		listenAddr:  ":" + port,
+		upstreamURL: "http://224.0.0.1",
+	}, 1, "unhealthy: connect to the app: ")
 
 	stop()
 
@@ -71,7 +76,10 @@ func TestHealthCheck(t *testing.T) {
 		t.Fatal("still running after being told to stop")
 	}
 
-	wantHealthCheck(t, env, 1, "unhealthy: ask smallwebwaf: ")
+	// Listening on port 0 takes any free port, so nothing listens on port 0
+	// itself.
+	wantHealthCheck(t, map[string]string{listenAddr: ":0"}, 1,
+		"unhealthy: ask smallwebwaf: ")
 	wantHealthCheck(t, map[string]string{listenAddr: "8080"}, 1,
 		"unhealthy: invalid setting: SWWAF_LISTEN_ADDR: ")
 }