diff --git a/lib/util/fs/index.ts b/lib/util/fs/index.ts
index eae38c5237599da760d904eec415ea8d5f407a89..114ed425b0c882672ff60ff3fa52bd3d6b12dedf 100644
--- a/lib/util/fs/index.ts
+++ b/lib/util/fs/index.ts
@@ -30,11 +30,13 @@ export async function readLocalFile(
 export async function readLocalFile(
   fileName: string,
   encoding?: string
-): Promise<string | Buffer> {
+): Promise<string | Buffer | null> {
   const { localDir } = GlobalConfig.get();
   const localFileName = join(localDir, fileName);
   try {
-    const fileContent = await fs.readFile(localFileName, encoding);
+    const fileContent = encoding
+      ? await fs.readFile(localFileName, encoding)
+      : await fs.readFile(localFileName);
     return fileContent;
   } catch (err) {
     logger.trace({ err }, 'Error reading local file');
diff --git a/lib/util/fs/proxies.ts b/lib/util/fs/proxies.ts
index 3abe0f4624a97cffa77657bafaee5dc33d8f670e..4008ec28950772100ae6ae0dbf004dd741110105 100644
--- a/lib/util/fs/proxies.ts
+++ b/lib/util/fs/proxies.ts
@@ -23,7 +23,7 @@ export function readFile(
   fileName: string,
   encoding?: string
 ): Promise<string | Buffer> {
-  return fs.readFile(fileName, encoding);
+  return encoding ? fs.readFile(fileName, encoding) : fs.readFile(fileName);
 }
 
 // istanbul ignore next
diff --git a/tsconfig.strict.json b/tsconfig.strict.json
index 9815efcfbd2e8039ca456246b2be5f118bb7a53c..d2bb5af542a5e3c6463e61c4944299d116b8a7a4 100644
--- a/tsconfig.strict.json
+++ b/tsconfig.strict.json
@@ -28,6 +28,7 @@
     "lib/util/date.ts",
     "lib/util/exec/common.ts",
     "lib/util/exec/types.ts",
+    "lib/util/fs",
     "lib/util/git/config.ts",
     "lib/util/git/types.ts",
     "lib/util/host-rules.ts",
@@ -66,6 +67,7 @@
     "lib/datasource/helm/common.ts",
     "lib/logger/err-serializer.spec.ts",
     "lib/manager/gradle/shallow/types.ts",
-    "lib/util/cache/*.spec.ts"
+    "lib/util/cache/*.spec.ts",
+    "lib/util/fs/index.spec.ts"
   ]
 }