diff --git a/lib/types/errors/external-host-error.ts b/lib/types/errors/external-host-error.ts
index 88f4fdb936f19f8d9c09bbc42ac654ea7176e752..5fd5922966a704f6879fe7ba79de01c2813d4606 100644
--- a/lib/types/errors/external-host-error.ts
+++ b/lib/types/errors/external-host-error.ts
@@ -1,7 +1,7 @@
 import { EXTERNAL_HOST_ERROR } from '../../constants/error-messages';
 
 export class ExternalHostError extends Error {
-  hostType: string;
+  hostType: string | undefined;
 
   err: Error;
 
diff --git a/lib/util/mask.spec.ts b/lib/util/mask.spec.ts
index a3150839d138d94b5ebe4642e1e897ea78e7a2dc..e7cdb0816c546436b5ab01523996bd1d0837e37f 100644
--- a/lib/util/mask.spec.ts
+++ b/lib/util/mask.spec.ts
@@ -2,7 +2,8 @@ import { maskToken } from './mask';
 
 describe('util/mask', () => {
   describe('.maskToken', () => {
-    it('returns value if passed value is falsy', () => {
+    it('returns empty string if passed value is falsy', () => {
+      expect(maskToken()).toBe('');
       expect(maskToken('')).toBe('');
     });
 
diff --git a/lib/util/mask.ts b/lib/util/mask.ts
index dd50d4336628d41bb514cc45fd911d3c2df4d645..83fa009a351b00edd7328288aaf1ccbdd4d2baa9 100644
--- a/lib/util/mask.ts
+++ b/lib/util/mask.ts
@@ -5,5 +5,5 @@ export function maskToken(str?: string): string {
         new Array(str.length - 3).join('*'),
         str.slice(-2),
       ].join('')
-    : str;
+    : '';
 }
diff --git a/test/graphql-snapshot.ts b/test/graphql-snapshot.ts
index 66e7f9f3eb378db61caa52fe47f2e97fcab4a00f..75478315b40f92b9d214729e8fd34763d1913a3e 100644
--- a/test/graphql-snapshot.ts
+++ b/test/graphql-snapshot.ts
@@ -37,11 +37,13 @@ type Variables = Record<string, string>;
 interface SelectionSet {
   __vars?: Variables;
   __args?: Arguments;
-  [key: string]: null | SelectionSet | Arguments;
+  [key: string]: undefined | null | SelectionSet | Arguments;
 }
 
 interface GraphqlSnapshot {
   query?: SelectionSet;
+  mutation?: SelectionSet;
+  subscription?: SelectionSet;
   variables?: Record<string, string>;
 }
 
@@ -81,7 +83,7 @@ function getArguments(key: string, val: ValueNode): Arguments {
 }
 
 function simplifyArguments(
-  argNodes: ReadonlyArray<ArgumentNode>
+  argNodes?: ReadonlyArray<ArgumentNode>
 ): Arguments | null {
   if (argNodes) {
     let result: Arguments = {};
diff --git a/tsconfig.strict.json b/tsconfig.strict.json
index efec9fbfe0faba954cf0c239975a2846e77adedf..50bda1274807772b4df58bb328bb924af685d210 100644
--- a/tsconfig.strict.json
+++ b/tsconfig.strict.json
@@ -88,6 +88,7 @@
     "./lib/platform/utils/pr-body.ts",
     "./lib/proxy.ts",
     "./lib/types/branch-status.ts",
+    "./lib/types/errors/external-host-error.ts",
     "./lib/types/git.ts",
     "./lib/types/host-rules.ts",
     "./lib/types/later.d.ts",
@@ -108,12 +109,15 @@
     "./lib/util/http/types.ts",
     "./lib/util/index.ts",
     "./lib/util/json-writer/indentation-type.ts",
+    "./lib/util/mask.spec.ts",
+    "./lib/util/mask.ts",
     "./lib/util/object.ts",
     "./lib/util/sanitize.ts",
     "./lib/util/split.ts",
     "./lib/workers/pr/changelog/hbs-template.ts",
     "./lib/workers/pr/changelog/types.ts",
     "./lib/workers/repository/init/types.ts",
+    "./test/graphql-snapshot.ts",
     "./test/json-schema.ts",
     "./test/newline-snapshot-serializer.ts",
     "./test/static-files.spec.ts",