diff --git a/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller.go b/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller.go
index 9d4d9acb534..4041d9c4355 100644
--- a/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller.go
+++ b/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller.go
@@ -90,6 +90,12 @@ func calculateCondition(in *apiextensionsv1.CustomResourceDefinition) *apiextens
 
 	allErrs := field.ErrorList{}
 
+	if in.Spec.PreserveUnknownFields {
+		allErrs = append(allErrs, field.Invalid(field.NewPath("spec", "preserveUnknownFields"),
+			in.Spec.PreserveUnknownFields,
+			fmt.Sprint("must be false")))
+	}
+
 	for i, v := range in.Spec.Versions {
 		if v.Schema == nil || v.Schema.OpenAPIV3Schema == nil {
 			continue
diff --git a/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller_test.go b/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller_test.go
new file mode 100644
index 00000000000..4b79f554ef5
--- /dev/null
+++ b/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/nonstructuralschema/nonstructuralschema_controller_test.go
@@ -0,0 +1,66 @@
+/*
+Copyright 2020 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package nonstructuralschema
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	apiextensionsv1 "k8s.io/apiextensions-apiserver/pkg/apis/apiextensions/v1"
+	"k8s.io/apimachinery/pkg/util/validation/field"
+)
+
+func Test_calculateCondition(t *testing.T) {
+	tests := []struct {
+		name string
+		args *apiextensionsv1.CustomResourceDefinition
+		want *apiextensionsv1.CustomResourceDefinitionCondition
+	}{
+		{
+			name: "preserve unknown fields is false",
+			args: &apiextensionsv1.CustomResourceDefinition{
+				Spec: apiextensionsv1.CustomResourceDefinitionSpec{
+					PreserveUnknownFields: false,
+				},
+			},
+		},
+		{
+			name: "preserve unknown fields is true",
+			args: &apiextensionsv1.CustomResourceDefinition{
+				Spec: apiextensionsv1.CustomResourceDefinitionSpec{
+					PreserveUnknownFields: true,
+				},
+			},
+			want: &apiextensionsv1.CustomResourceDefinitionCondition{
+				Type:   apiextensionsv1.NonStructuralSchema,
+				Status: apiextensionsv1.ConditionTrue,
+				Reason: "Violations",
+				Message: field.Invalid(field.NewPath("spec", "preserveUnknownFields"),
+					true,
+					fmt.Sprint("must be false")).Error(),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := calculateCondition(tt.args); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("calculateCondition() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}