Ensure that FormWizard, TabContent and WizardStep class declarations extend Vue.

types/FormWizard.ts

@@ -1,8 +1,10 @@
+import Vue from 'vue'
+
 export type ShapeType = 'circle' | 'square' | 'tab'
 export type LayoutType = 'vertical' | 'horizontal'
 export type StepSizeType = 'xs' | 'sm' | 'md' | 'lg'
 
-export declare class Wizard {
+export declare class Wizard extends Vue {
   /**
    * Wizard identifier.
    * 

types/TabContent.ts

@@ -1,4 +1,6 @@
-export declare class Tab {
+import Vue from 'vue'
+
+export declare class Tab extends Vue {
   /** Title to be displayed under each step */
   title?: string
   /** css class for each step icon */

types/WizardStep.ts

@@ -1,4 +1,6 @@
-export declare class Step {
+import Vue from 'vue'
+
+export declare class Step extends Vue {
   /** Tab component for the step. This should be usually passed through the `step` scoped slot */
   tab: object
   transition: string