diff --git a/src/app/converter-registry.service.spec.ts b/src/app/converter-registry.service.spec.ts
index 2b97a17..e0cfa46 100644
--- a/src/app/converter-registry.service.spec.ts
+++ b/src/app/converter-registry.service.spec.ts
@@ -1,15 +1,40 @@
-import { TestBed, inject } from '@angular/core/testing';
+import {inject, TestBed} from '@angular/core/testing';
 
-import { ConverterRegistryService } from './converter-registry.service';
+import {ConverterRegistryService} from './converter-registry.service';
+import {NativeLibraryWrapperService} from './native-library-wrapper.service';
+import {Converter} from './converter/converter';
+import {Base64Decoder} from './converter/base64-decoder';
+import createSpyObj = jasmine.createSpyObj;
 
 describe('ConverterRegistryService', () => {
   beforeEach(() => {
     TestBed.configureTestingModule({
-      providers: [ConverterRegistryService]
+      providers: [
+        ConverterRegistryService,
+        {provide: NativeLibraryWrapperService, useValue: createSpyObj(['punycode', 'quotedPrintable', 'utf8'])}
+      ]
     });
   });
 
   it('should be created', inject([ConverterRegistryService], (service: ConverterRegistryService) => {
     expect(service).toBeTruthy();
   }));
+
+  it('should register converters upon creation', inject([ConverterRegistryService], (service: ConverterRegistryService) => {
+    expect(service.getAllConverters()).toBeTruthy();
+    expect(service.getAllConverters().length).toBeGreaterThan(0);
+  }));
+
+  it('must not allow the same converter ID to be regisgered more than once',
+    inject([ConverterRegistryService], (service: ConverterRegistryService) => {
+      // arrange
+      const duplicateConverter: Converter = new Base64Decoder();
+      const duplicateConverterId = duplicateConverter.getId();
+      expect(() => {
+        // act
+        (service as any).registerConverter(duplicateConverter);
+      })
+      // assert
+        .toThrowError(`Converter-ID ${duplicateConverterId} is already registered!`);
+    }));
 });
diff --git a/src/app/native-library-wrapper.service.ts b/src/app/native-library-wrapper.service.ts
index 308a14f..eb661b1 100644
--- a/src/app/native-library-wrapper.service.ts
+++ b/src/app/native-library-wrapper.service.ts
@@ -7,9 +7,9 @@ import * as NativeUtf8 from 'utf8';
   providedIn: 'root'
 })
 export class NativeLibraryWrapperService {
-  public utf8: Utf8;
-  public quotedPrintable: QuotedPrintable;
-  public punycode: Punycode;
+  public readonly utf8: Utf8;
+  public readonly quotedPrintable: QuotedPrintable;
+  public readonly punycode: Punycode;
 
   constructor() {
     this.utf8 = NativeUtf8;