diff --git a/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/defaulters_suite_test.go b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/defaulters_suite_test.go
index 35c9398e98..ec397d8b02 100644
--- a/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/defaulters_suite_test.go
+++ b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/defaulters_suite_test.go
@@ -97,7 +97,7 @@ var _ = Describe("Set default class in virtualMachineClasName", func() {
 	})
 
 	Context("creating VM with empty virtualMachineClassName", func() {
-		It("should keep virtualMachineClassName empty if no default class", func() {
+		It("should return a clear error if no default class", func() {
 			// Initialize fake client with some classes.
 			name := "single-custom-class"
 			setup(
@@ -107,7 +107,7 @@ var _ = Describe("Set default class in virtualMachineClasName", func() {
 
 			vm := newVMWithEmptyClass("vm-with-empty-class")
 			err := classDefaulter.Default(ctx, vm)
-			Expect(err).Should(BeNil())
+			Expect(err).Should(MatchError(ContainSubstring("spec.virtualMachineClassName is empty and no default VirtualMachineClass is configured")))
 			Expect(vm.Spec.VirtualMachineClassName).Should(BeEmpty())
 		})
 
diff --git a/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/virtual_machine_class_name.go b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/virtual_machine_class_name.go
index 834afb6f5f..ff826aed23 100644
--- a/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/virtual_machine_class_name.go
+++ b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/virtual_machine_class_name.go
@@ -22,6 +22,7 @@ import (
 
 	"sigs.k8s.io/controller-runtime/pkg/client"
 
+	"github.com/deckhouse/virtualization-controller/pkg/common/annotations"
 	"github.com/deckhouse/virtualization-controller/pkg/controller/service"
 	"github.com/deckhouse/virtualization/api/core/v1alpha2"
 )
@@ -56,10 +57,8 @@ func (v *VirtualMachineClassNameDefaulter) Default(ctx context.Context, vm *v1al
 		return err
 	}
 
-	// "No default class" is not a mutating error, validators will complain
-	// about missing field during validation phase later.
 	if defaultClass == nil {
-		return nil
+		return fmt.Errorf("spec.virtualMachineClassName is empty and no default VirtualMachineClass is configured; set spec.virtualMachineClassName explicitly or mark one VirtualMachineClass as default with the %q annotation set to %q", annotations.AnnVirtualMachineClassDefault, "true")
 	}
 
 	vm.Spec.VirtualMachineClassName = defaultClass.GetName()