diff --git a/internal/webhooks/cluster.go b/internal/webhooks/cluster.go
index 1abfa1d7e927..087bb0e14c8c 100644
--- a/internal/webhooks/cluster.go
+++ b/internal/webhooks/cluster.go
@@ -101,6 +101,18 @@ func (webhook *Cluster) Default(ctx context.Context, obj runtime.Object) error {
 		if !strings.HasPrefix(cluster.Spec.Topology.Version, "v") {
 			cluster.Spec.Topology.Version = "v" + cluster.Spec.Topology.Version
 		}
+
+		if cluster.Spec.Topology.Class == "" {
+			allErrs = append(
+				allErrs,
+				field.Required(
+					field.NewPath("spec", "topology", "class"),
+					"class cannot be empty",
+				),
+			)
+			return apierrors.NewInvalid(clusterv1.GroupVersion.WithKind("Cluster").GroupKind(), cluster.Name, allErrs)
+		}
+
 		clusterClass, err := webhook.pollClusterClassForCluster(ctx, cluster)
 		if err != nil {
 			// If the ClusterClass can't be found or is not up to date ignore the error.
diff --git a/internal/webhooks/cluster_test.go b/internal/webhooks/cluster_test.go
index cbe948d0d444..02c066287e8a 100644
--- a/internal/webhooks/cluster_test.go
+++ b/internal/webhooks/cluster_test.go
@@ -897,6 +897,23 @@ func TestClusterDefaultTopologyVersion(t *testing.T) {
 	g.Expect(c.Spec.Topology.Version).To(HavePrefix("v"))
 }
 
+func TestClusterFailOnMissingClassField(t *testing.T) {
+	// NOTE: ClusterTopology feature flag is disabled by default, thus preventing to set Cluster.Topologies.
+	// Enabling the feature flag temporarily for this test.
+	defer utilfeature.SetFeatureGateDuringTest(t, feature.Gates, feature.ClusterTopology, true)()
+
+	g := NewWithT(t)
+
+	c := builder.Cluster("fooboo", "cluster1").
+		WithTopology(builder.ClusterTopology().
+			WithClass(""). // THis is invalid.
+			WithVersion("1.19.1").
+			Build()).
+		Build()
+
+	g.Expect((&Cluster{}).Default(ctx, c)).ToNot(Succeed())
+}
+
 func TestClusterValidation(t *testing.T) {
 	// NOTE: ClusterTopology feature flag is disabled by default, thus preventing to set Cluster.Topologies.