diff --git a/ts/classes.xinvoice.ts b/ts/classes.xinvoice.ts
index 73c3081..c728d8d 100644
--- a/ts/classes.xinvoice.ts
+++ b/ts/classes.xinvoice.ts
@@ -22,29 +22,29 @@ import { BaseValidator } from './formats/base.validator.js';
  */
 export class XInvoice implements plugins.tsclass.business.ILetter {
   // ILetter interface properties
-  public versionInfo: { type: string; version: string } = {
+  public versionInfo: plugins.tsclass.business.ILetter['versionInfo'] = {
     type: 'draft',
     version: '1.0.0'
   };
-  public type: string = 'invoice';
-  public date: number = Date.now();
-  public subject: string = '';
-  public from: plugins.tsclass.business.IContact;
-  public to: plugins.tsclass.business.IContact;
+  public type: plugins.tsclass.business.ILetter['type'] = 'invoice';
+  public date = Date.now();
+  public subject: plugins.tsclass.business.ILetter['subject'] = '';
+  public from: plugins.tsclass.business.TContact;
+  public to: plugins.tsclass.business.TContact;
   public content: {
     invoiceData: plugins.tsclass.finance.IInvoice;
     textData: null;
     timesheetData: null;
     contractData: null;
   };
-  public needsCoverSheet: boolean = false;
-  public objectActions: any[] = [];
-  public pdf: Uint8Array | null = null;
-  public incidenceId: null = null;
-  public language: string | null = null;
-  public legalContact: any | null = null;
-  public logoUrl: string | null = null;
-  public pdfAttachments: any | null = null;
+  public needsCoverSheet: plugins.tsclass.business.ILetter['needsCoverSheet'] = false;
+  public objectActions: plugins.tsclass.business.ILetter['objectActions'] = [];
+  public pdf: plugins.tsclass.business.ILetter['pdf'] = null;
+  public incidenceId: plugins.tsclass.business.ILetter['incidenceId'] = null;
+  public language: plugins.tsclass.business.ILetter['language'] = null;
+  public legalContact: plugins.tsclass.business.ILetter['legalContact'] = null;
+  public logoUrl: plugins.tsclass.business.ILetter['logoUrl'] = null;
+  public pdfAttachments: plugins.tsclass.business.ILetter['pdfAttachments'] = null;
   public accentColor: string | null = null;
 
   // XInvoice specific properties
@@ -92,7 +92,7 @@ export class XInvoice implements plugins.tsclass.business.ILetter {
   /**
    * Creates an empty IContact object
    */
-  private createEmptyContact(): plugins.tsclass.business.IContact {
+  private createEmptyContact(): plugins.tsclass.business.TContact {
     return {
       name: '',
       type: 'company',