diff --git a/pkg/promunifi/collector.go b/pkg/promunifi/collector.go
index 9b82630b..4a2ecc11 100644
--- a/pkg/promunifi/collector.go
+++ b/pkg/promunifi/collector.go
@@ -234,7 +234,7 @@ func (u *promUnifi) DebugOutput() (bool, error) {
 	}
 
 	if u.HTTPListen == "" {
-		return false, fmt.Errorf("invalid listen string")
+		u.HTTPListen = defaultHTTPListen
 	}
 
 	// check the port
diff --git a/pkg/promunifi/collector_test.go b/pkg/promunifi/collector_test.go
new file mode 100644
index 00000000..113ac66e
--- /dev/null
+++ b/pkg/promunifi/collector_test.go
@@ -0,0 +1,22 @@
+//nolint:testpackage // white-box: exercises the unexported output plugin.
+package promunifi
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/unpoller/unpoller/pkg/poller"
+)
+
+func TestDebugOutputAcceptsUnsetHTTPListen(t *testing.T) {
+	poller.SetHealthCheckMode(true)
+	t.Cleanup(func() { poller.SetHealthCheckMode(false) })
+
+	u := &promUnifi{Config: &Config{}}
+
+	ok, err := u.DebugOutput()
+	require.NoError(t, err)
+	assert.True(t, ok)
+	assert.Equal(t, defaultHTTPListen, u.HTTPListen)
+}