diff --git a/internal/analytics/collector.go b/internal/analytics/collector.go
index 859ec423e..bea1eb39c 100644
--- a/internal/analytics/collector.go
+++ b/internal/analytics/collector.go
@@ -60,14 +60,15 @@ type aggregateData struct {
 	commitment        float64
 	commitmentUnknown bool
 	// usage accumulates the recurring (monthly) cost of the commitments in this
-	// bucket as the covered-usage proxy. usageKnown stays false until at least
-	// one contributing row carried a non-nil MonthlyCost, so a bucket made up
-	// entirely of all-upfront commitments writes NULL usage rather than 0
+	// bucket as the covered-usage proxy. usageUnknown is set as soon as one
+	// contributing row has a nil MonthlyCost, exactly like commitmentUnknown,
+	// so a bucket with any all-upfront commitment writes NULL usage rather
+	// than a partial sum marked as known (issue #1423). A known 0 stays 0
 	// (feedback_nullable_not_zero).
-	usage      float64
-	usageKnown bool
-	savings    float64
-	count      int
+	usage        float64
+	usageUnknown bool
+	savings      float64
+	count        int
 }
 
 // aggKey is the bucket identity. cloudAccountID is dereferenced (or "" when
@@ -185,12 +186,13 @@ func aggregatePurchases(ctx context.Context, purchases []config.PurchaseHistoryR
 		} else {
 			agg.commitment += *p.UpfrontCost / (float64(p.Term) * MonthsPerYear)
 		}
-		// H2: real covered usage from the recurring monthly cost when present.
-		// Nil MonthlyCost (e.g. AWS all-upfront) contributes nothing and leaves
-		// usage unknown rather than implicitly $0.
-		if p.MonthlyCost != nil {
+		// H2: real covered usage from the recurring monthly cost. One nil
+		// MonthlyCost (e.g. AWS all-upfront) makes the whole bucket's usage
+		// unknown, rather than implicitly $0 for that row.
+		if p.MonthlyCost == nil {
+			agg.usageUnknown = true
+		} else {
 			agg.usage += *p.MonthlyCost
-			agg.usageKnown = true
 		}
 		agg.count++
 	}
@@ -208,7 +210,7 @@ func commitmentTypeFor(service string) string {
 }
 
 // buildSnapshots converts the aggregation map into snapshot rows. total_usage is
-// nil when no contributing row carried a recurring cost; coverage_percentage is
+// nil when any contributing row lacks a recurring cost; coverage_percentage is
 // nil because purchase_history carries no on-demand baseline to derive it from
 // (writing a placeholder 0 would corrupt AVG, per feedback_nullable_not_zero).
 func buildSnapshots(serviceMap map[string]*aggregateData, now time.Time) []SavingsSnapshot {
@@ -219,7 +221,7 @@ func buildSnapshots(serviceMap map[string]*aggregateData, now time.Time) []Savin
 			commitment = &agg.commitment
 		}
 		var usage *float64
-		if agg.usageKnown {
+		if !agg.usageUnknown {
 			u := agg.usage
 			usage = &u
 		}
diff --git a/internal/analytics/nullable_purchase_cost_test.go b/internal/analytics/nullable_purchase_cost_test.go
index 2f3b2318e..7fb9346e7 100644
--- a/internal/analytics/nullable_purchase_cost_test.go
+++ b/internal/analytics/nullable_purchase_cost_test.go
@@ -48,3 +48,52 @@ func TestCollectorUnknownUpfront(t *testing.T) {
 		}
 	}
 }
+
+// TestCollectorUnknownMonthlyCost covers issue #1423: one nil MonthlyCost in a
+// snapshot row makes total_usage unknown instead of a partial sum stored as
+// known, while a row whose costs are all known (including a known 0) keeps its
+// real total.
+func TestCollectorUnknownMonthlyCost(t *testing.T) {
+	cases := []struct {
+		name  string
+		costs []*float64
+		want  *float64
+	}{
+		{"recurring then all-upfront", []*float64{new(float64(120)), nil}, nil},
+		{"all-upfront then recurring", []*float64{nil, new(float64(120))}, nil},
+		{"all unknown", []*float64{nil, nil}, nil},
+		{"all known", []*float64{new(float64(120)), new(float64(30))}, new(float64(150))},
+		{"known zero stays zero", []*float64{new(float64)}, new(float64)},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			purchases := make([]config.PurchaseHistoryRecord, len(tc.costs), len(tc.costs)+1)
+			for i, cost := range tc.costs {
+				purchases[i] = activeRecord("aws", "ec2", "us-east-1", 1, 10, 0)
+				purchases[i].MonthlyCost = cost
+			}
+			other := activeRecord("aws", "ec2", "us-east-1", 1, 20, 240)
+			other.AccountID = "other-account"
+			other.MonthlyCost = new(float64(45))
+			purchases = append(purchases, other)
+			groups, _, _, err := aggregatePurchases(context.Background(), purchases, time.Now())
+			require.NoError(t, err)
+			snapshots := buildSnapshots(groups, time.Now())
+			require.Len(t, snapshots, 2)
+			for _, snapshot := range snapshots {
+				if snapshot.AccountID == other.AccountID {
+					require.NotNil(t, snapshot.TotalUsage, "an unknown cost must not leak into another row")
+					assert.InDelta(t, 45.0, *snapshot.TotalUsage, 0.001)
+					continue
+				}
+				assert.Equal(t, len(tc.costs), snapshot.Metadata["active_purchases"])
+				if tc.want == nil {
+					assert.Nil(t, snapshot.TotalUsage)
+					continue
+				}
+				require.NotNil(t, snapshot.TotalUsage)
+				assert.InDelta(t, *tc.want, *snapshot.TotalUsage, 0.001)
+			}
+		})
+	}
+}