diff --git a/pkg/api/resource/quantity.go b/pkg/api/resource/quantity.go
index f57acbb9fee..4c6d669e523 100644
--- a/pkg/api/resource/quantity.go
+++ b/pkg/api/resource/quantity.go
@@ -328,15 +328,28 @@ func (q *Quantity) Cmp(y Quantity) int {
 }
 
 func (q *Quantity) Add(y Quantity) error {
-	q.Amount.Add(q.Amount, y.Amount)
+	switch {
+	case y.Amount == nil:
+		// Adding 0: do nothing.
+	case q.Amount == nil:
+		q.Amount = &inf.Dec{}
+		return q.Add(y)
+	default:
+		q.Amount.Add(q.Amount, y.Amount)
+	}
 	return nil
 }
 
 func (q *Quantity) Sub(y Quantity) error {
-	if q.Format != y.Format {
-		return fmt.Errorf("format mismatch: %v vs. %v", q.Format, y.Format)
+	switch {
+	case y.Amount == nil:
+		// Subtracting 0: do nothing.
+	case q.Amount == nil:
+		q.Amount = &inf.Dec{}
+		return q.Sub(y)
+	default:
+		q.Amount.Sub(q.Amount, y.Amount)
 	}
-	q.Amount.Sub(q.Amount, y.Amount)
 	return nil
 }
 
diff --git a/pkg/api/resource/quantity_test.go b/pkg/api/resource/quantity_test.go
index da8858ea97d..c600294c60d 100644
--- a/pkg/api/resource/quantity_test.go
+++ b/pkg/api/resource/quantity_test.go
@@ -535,3 +535,47 @@ func TestQFlagIsPFlag(t *testing.T) {
 		t.Errorf("Unexpected result %v != %v", e, a)
 	}
 }
+
+func TestSub(t *testing.T) {
+	tests := []struct {
+		a        Quantity
+		b        Quantity
+		expected Quantity
+	}{
+		{Quantity{dec(10, 0), DecimalSI}, Quantity{dec(1, 1), DecimalSI}, Quantity{dec(0, 0), DecimalSI}},
+		{Quantity{dec(10, 0), DecimalSI}, Quantity{dec(1, 0), BinarySI}, Quantity{dec(9, 0), DecimalSI}},
+		{Quantity{dec(10, 0), BinarySI}, Quantity{dec(1, 0), DecimalSI}, Quantity{dec(9, 0), BinarySI}},
+		{Quantity{nil, DecimalSI}, Quantity{dec(50, 0), DecimalSI}, Quantity{dec(-50, 0), DecimalSI}},
+		{Quantity{dec(50, 0), DecimalSI}, Quantity{nil, DecimalSI}, Quantity{dec(50, 0), DecimalSI}},
+		{Quantity{nil, DecimalSI}, Quantity{nil, DecimalSI}, Quantity{dec(0, 0), DecimalSI}},
+	}
+
+	for i, test := range tests {
+		test.a.Sub(test.b)
+		if test.a.Cmp(test.expected) != 0 {
+			t.Errorf("[%d] Expected %q, got %q", i, test.expected.String(), test.a.String())
+		}
+	}
+}
+
+func TestAdd(t *testing.T) {
+	tests := []struct {
+		a        Quantity
+		b        Quantity
+		expected Quantity
+	}{
+		{Quantity{dec(10, 0), DecimalSI}, Quantity{dec(1, 1), DecimalSI}, Quantity{dec(20, 0), DecimalSI}},
+		{Quantity{dec(10, 0), DecimalSI}, Quantity{dec(1, 0), BinarySI}, Quantity{dec(11, 0), DecimalSI}},
+		{Quantity{dec(10, 0), BinarySI}, Quantity{dec(1, 0), DecimalSI}, Quantity{dec(11, 0), BinarySI}},
+		{Quantity{nil, DecimalSI}, Quantity{dec(50, 0), DecimalSI}, Quantity{dec(50, 0), DecimalSI}},
+		{Quantity{dec(50, 0), DecimalSI}, Quantity{nil, DecimalSI}, Quantity{dec(50, 0), DecimalSI}},
+		{Quantity{nil, DecimalSI}, Quantity{nil, DecimalSI}, Quantity{dec(0, 0), DecimalSI}},
+	}
+
+	for i, test := range tests {
+		test.a.Add(test.b)
+		if test.a.Cmp(test.expected) != 0 {
+			t.Errorf("[%d] Expected %q, got %q", i, test.expected.String(), test.a.String())
+		}
+	}
+}