diff --git a/apps/api/src/helper/bearer-token.helper.ts b/apps/api/src/helper/bearer-token.helper.ts
index 2c7f9cd65e..d9bc82c43e 100644
--- a/apps/api/src/helper/bearer-token.helper.ts
+++ b/apps/api/src/helper/bearer-token.helper.ts
@@ -40,8 +40,9 @@ export function getActiveImpersonationOfBearerToken(
  * hence the transport lists only the tools which the access covers.
  */
 export function getMcpUserOfBearerToken(request: unknown) {
-  const impersonationOfBearerToken =
-    getActiveImpersonationOfBearerToken(request);
+  const impersonationOfBearerToken = getActiveImpersonationOfBearerToken(
+    request as Pick<RequestWithUser, 'impersonationOfBearerToken'>
+  );
 
   return impersonationOfBearerToken
     ? { scopes: impersonationOfBearerToken.scopes }
@@ -54,8 +55,9 @@ export function getMcpUserOfBearerToken(request: unknown) {
  * they apply only to an access which reads the monetary values.
  */
 export function hasLimitsOfBasicSubscriptionOfBearerToken(request: unknown) {
-  const impersonationOfBearerToken =
-    getActiveImpersonationOfBearerToken(request);
+  const impersonationOfBearerToken = getActiveImpersonationOfBearerToken(
+    request as Pick<RequestWithUser, 'impersonationOfBearerToken'>
+  );
 
   return (
     impersonationOfBearerToken?.userSubscription?.type ===
diff --git a/apps/api/src/helper/country.helper.ts b/apps/api/src/helper/country.helper.ts
index 119f5e44c1..9523dec339 100644
--- a/apps/api/src/helper/country.helper.ts
+++ b/apps/api/src/helper/country.helper.ts
@@ -9,7 +9,7 @@ export function getCountryCodeByName({
   aliases?: Record<string, string>;
   dataSource?: string;
   name: string;
-}): string {
+}): string | undefined {
   if (aliases[name]) {
     return aliases[name];
   }
diff --git a/apps/api/src/helper/data-source.helper.spec.ts b/apps/api/src/helper/data-source.helper.spec.ts
index 3bfef82add..68b578c92e 100644
--- a/apps/api/src/helper/data-source.helper.spec.ts
+++ b/apps/api/src/helper/data-source.helper.spec.ts
@@ -29,9 +29,9 @@ function createConfigurationService({
 describe('DataSourceHelper', () => {
   describe('isValidEncodedDataSource', () => {
     it('Accepts an encoded data source', () => {
-      expect(isValidEncodedDataSource(encodeDataSource(DataSource.YAHOO))).toBe(
-        true
-      );
+      expect(
+        isValidEncodedDataSource(encodeDataSource(DataSource.YAHOO) ?? '')
+      ).toBe(true);
     });
 
     it('Refuses a data source which is not encoded', () => {
diff --git a/apps/api/src/helper/data-source.helper.ts b/apps/api/src/helper/data-source.helper.ts
index fb0ded3f1f..831705e5ca 100644
--- a/apps/api/src/helper/data-source.helper.ts
+++ b/apps/api/src/helper/data-source.helper.ts
@@ -64,7 +64,7 @@ function hashDataSource(dataSource: DataSource) {
   return createHash('sha256').update(dataSource).digest('hex').slice(0, 8);
 }
 
-export function decodeDataSource(encodedDataSource: string) {
+export function decodeDataSource(encodedDataSource?: string) {
   if (!encodedDataSource) {
     return undefined;
   }
@@ -128,7 +128,7 @@ export function transformDataSourceInRequest({
   configurationService: ConfigurationService;
   dataSource?: string;
 }) {
-  if (Object.hasOwn(DataSource, dataSource)) {
+  if (dataSource && Object.hasOwn(DataSource, dataSource)) {
     return getUnmaskedGhostfolioDataSource({
       dataSource: dataSource as DataSource,
       ghostfolioDataSources: getGhostfolioDataSources({ configurationService })
diff --git a/apps/api/src/helper/markdown-table.helper.ts b/apps/api/src/helper/markdown-table.helper.ts
index 3bc8a7a8c7..26979cde3d 100644
--- a/apps/api/src/helper/markdown-table.helper.ts
+++ b/apps/api/src/helper/markdown-table.helper.ts
@@ -17,7 +17,7 @@ export function getTableInput<T, C = void>({
     rows: rows.map((row) => {
       return columnDefinitions.reduce(
         (tableRow, { getValue, name }) => {
-          tableRow[name] = getValue(row, context);
+          tableRow[name] = getValue(row, context as C);
 
           return tableRow;
         },
diff --git a/apps/api/src/helper/portfolio.helper.ts b/apps/api/src/helper/portfolio.helper.ts
index b8152414bb..f91ab0f281 100644
--- a/apps/api/src/helper/portfolio.helper.ts
+++ b/apps/api/src/helper/portfolio.helper.ts
@@ -21,7 +21,7 @@ export function convertValuesToPercentages({
 
   const totalValue = holdings
     .map(({ valueInBaseCurrency }) => {
-      return valueInBaseCurrency;
+      return valueInBaseCurrency ?? 0;
     })
     .reduce((a, b) => {
       return a + b;
@@ -32,7 +32,7 @@ export function convertValuesToPercentages({
       ? holding.investment / totalInvestment
       : 0;
     holding.valueInPercentage = totalValue
-      ? holding.valueInBaseCurrency / totalValue
+      ? (holding.valueInBaseCurrency ?? 0) / totalValue
       : 0;
   }
 
diff --git a/apps/api/src/helper/string.helper.ts b/apps/api/src/helper/string.helper.ts
index 75f9d00fd0..4acb4cb235 100644
--- a/apps/api/src/helper/string.helper.ts
+++ b/apps/api/src/helper/string.helper.ts
@@ -3,7 +3,7 @@ import { randomBytes } from 'node:crypto';
 export function getRandomString(length: number) {
   const bytes = randomBytes(length);
   const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
-  const result = [];
+  const result: string[] = [];
 
   for (let i = 0; i < length; i++) {
     const randomByte = bytes[i];