diff --git a/internal/api/incidents_test.go b/internal/api/incidents_test.go
index 2022609..4820503 100644
--- a/internal/api/incidents_test.go
+++ b/internal/api/incidents_test.go
@@ -653,6 +653,61 @@ func TestStats_Incidents(t *testing.T) {
 	}
 }
 
+// An empty window is a report of zero, not a failure. SUM over no rows is NULL
+// in SQLite, which used to come back as a 500 the moment every incident was
+// archived — the state a quiet installation settles into.
+func TestStats_IncidentsEmptyWindowIsZeroNotAnError(t *testing.T) {
+	s := newTS(t)
+
+	// No incidents at all.
+	resp := s.req(t, http.MethodGet, "/api/stats/incidents", nil)
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		t.Fatalf("expected 200 on an empty database, got %d", resp.StatusCode)
+	}
+	var stats map[string]any
+	decode(t, resp, &stats)
+	for _, k := range []string{"total", "triggered", "acknowledged", "resolved"} {
+		if stats[k].(float64) != 0 {
+			t.Errorf("expected %s 0, got %v", k, stats[k])
+		}
+	}
+
+	// And with every incident archived out of the window.
+	postWebhook(t, s, []map[string]any{
+		amAlert("fp-s4", "Gone", "firing", "2026-05-20T10:00:00Z", zeroTime, nil),
+	})
+	s.req(t, http.MethodPost, "/api/incidents/1/archive", nil).Body.Close()
+
+	resp = s.req(t, http.MethodGet, "/api/stats/incidents", nil)
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		t.Fatalf("expected 200 when every incident is archived, got %d", resp.StatusCode)
+	}
+	stats = nil
+	decode(t, resp, &stats)
+	if stats["total"].(float64) != 0 {
+		t.Errorf("expected total 0, got %v", stats["total"])
+	}
+}
+
+// The alert stats share the same aggregate, and the same empty-window trap.
+func TestStats_AlertsEmptyWindowIsZeroNotAnError(t *testing.T) {
+	s := newTS(t)
+	resp := s.req(t, http.MethodGet, "/api/stats/alerts", nil)
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		t.Fatalf("expected 200 on an empty database, got %d", resp.StatusCode)
+	}
+	var stats map[string]any
+	decode(t, resp, &stats)
+	for _, k := range []string{"total", "firing", "resolved"} {
+		if stats[k].(float64) != 0 {
+			t.Errorf("expected %s 0, got %v", k, stats[k])
+		}
+	}
+}
+
 // Nothing acknowledged yet means "no data", which is not the same claim as zero.
 func TestStats_IncidentsNullMTTAWhenNothingAcknowledged(t *testing.T) {
 	s := newTS(t)
diff --git a/internal/api/stats.go b/internal/api/stats.go
index 7b20a2c..2572994 100644
--- a/internal/api/stats.go
+++ b/internal/api/stats.go
@@ -13,11 +13,14 @@ func handleStatsAlerts(db *sql.DB) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		where, args := statsFilter(r.URL.Query(), "received_at")
 
+		// COALESCE because SUM over zero rows is NULL, not 0, and a count of
+		// nothing is 0 — without it an empty window is a 500 rather than a
+		// legitimately empty report.
 		var total, firing, resolved int64
 		err := db.QueryRowContext(r.Context(), fmt.Sprintf(`
 			SELECT COUNT(*),
-			       SUM(CASE WHEN status = 'firing'   THEN 1 ELSE 0 END),
-			       SUM(CASE WHEN status = 'resolved' THEN 1 ELSE 0 END)
+			       COALESCE(SUM(CASE WHEN status = 'firing'   THEN 1 ELSE 0 END), 0),
+			       COALESCE(SUM(CASE WHEN status = 'resolved' THEN 1 ELSE 0 END), 0)
 			FROM alerts WHERE %s`, where), args...,
 		).Scan(&total, &firing, &resolved)
 		if err != nil {
@@ -167,13 +170,16 @@ func handleStatsIncidents(db *sql.DB) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		where, args := statsFilter(r.URL.Query(), "triggered_at")
 
+		// The counts are COALESCEd because SUM over zero rows is NULL, not 0.
+		// The averages are not: mtta and mttr stay null on purpose, since zero
+		// would read as "instant" rather than "nothing to measure yet".
 		var total, triggered, acknowledged, resolved int64
 		var mtta, mttr *float64
 		err := db.QueryRowContext(r.Context(), fmt.Sprintf(`
 			SELECT COUNT(*),
-			       SUM(CASE WHEN status = 'triggered'    THEN 1 ELSE 0 END),
-			       SUM(CASE WHEN status = 'acknowledged' THEN 1 ELSE 0 END),
-			       SUM(CASE WHEN status = 'resolved'     THEN 1 ELSE 0 END),
+			       COALESCE(SUM(CASE WHEN status = 'triggered'    THEN 1 ELSE 0 END), 0),
+			       COALESCE(SUM(CASE WHEN status = 'acknowledged' THEN 1 ELSE 0 END), 0),
+			       COALESCE(SUM(CASE WHEN status = 'resolved'     THEN 1 ELSE 0 END), 0),
 			       AVG(CASE WHEN acknowledged_at IS NOT NULL
 			                THEN acknowledged_at - triggered_at END),
 			       AVG(CASE WHEN resolved_at IS NOT NULL