diff --git a/golang/cosmos/x/swingset/types/params.go b/golang/cosmos/x/swingset/types/params.go
index 30b12d19be3..9b53ae58fbf 100644
--- a/golang/cosmos/x/swingset/types/params.go
+++ b/golang/cosmos/x/swingset/types/params.go
@@ -172,10 +172,20 @@ func validateQueueMax(i interface{}) error {
 }
 
 func validateVatCleanupBudget(i interface{}) error {
-	_, ok := i.([]UintMapEntry)
+	entries, ok := i.([]UintMapEntry)
 	if !ok {
 		return fmt.Errorf("invalid parameter type: %T", i)
 	}
+	hasDefault := false
+	for _, entry := range entries {
+		if entry.Key == VatCleanupDefault {
+			hasDefault = true
+			break
+		}
+	}
+	if len(entries) > 0 && !hasDefault {
+		return fmt.Errorf("`default` must be present in a non-empty vat cleanup budget")
+	}
 	return nil
 }
 
diff --git a/golang/cosmos/x/swingset/types/params_test.go b/golang/cosmos/x/swingset/types/params_test.go
index b43e08351f3..ff3a7cbe149 100644
--- a/golang/cosmos/x/swingset/types/params_test.go
+++ b/golang/cosmos/x/swingset/types/params_test.go
@@ -146,3 +146,37 @@ func TestUpdateParamsFromExisting(t *testing.T) {
 		t.Errorf("GOT\n%v\nWANTED\n%v", got, want)
 	}
 }
+
+func TestValidateParams(t *testing.T) {
+	params := Params{
+		BeansPerUnit:       DefaultBeansPerUnit(),
+		BootstrapVatConfig: "foo",
+		FeeUnitPrice:       sdk.NewCoins(sdk.NewInt64Coin("denom", 789)),
+		PowerFlagFees:      DefaultPowerFlagFees,
+		QueueMax:           DefaultQueueMax,
+		VatCleanupBudget:   DefaultVatCleanupBudget,
+	}
+	err := params.ValidateBasic()
+	if err != nil {
+		t.Errorf("unexpected ValidateBasic() error with default params: %v", err)
+	}
+
+	customVatCleanup := UintMapEntry{"corge", sdk.NewUint(4)}
+	params.VatCleanupBudget = append(params.VatCleanupBudget, customVatCleanup)
+	err = params.ValidateBasic()
+	if err != nil {
+		t.Errorf("unexpected ValidateBasic() error with extended params: %v", err)
+	}
+
+	params.VatCleanupBudget = params.VatCleanupBudget[1:]
+	err = params.ValidateBasic()
+	if err == nil {
+		t.Errorf("ValidateBasic() failed to reject VatCleanupBudget with missing `default` %v", params.VatCleanupBudget)
+	}
+
+	params.VatCleanupBudget = []UintMapEntry{}
+	err = params.ValidateBasic()
+	if err != nil {
+		t.Errorf("unexpected ValidateBasic() error with empty VatCleanupBudget: %v", params.VatCleanupBudget)
+	}
+}