diff --git a/src/verifyAndStoreOptionValue.ts b/src/verifyAndStoreOptionValue.ts
index c341cf0..4db5ab9 100644
--- a/src/verifyAndStoreOptionValue.ts
+++ b/src/verifyAndStoreOptionValue.ts
@@ -2,7 +2,7 @@ import IOption from './interfaces/IOption';
 import IState from './interfaces/IState';
 
 /** Calls the given function, and in case of an error, uses the given function to provide a custom description for the error message. */
-function execWithCustomError(func: (...args: any[]) => any, showError: (error: Error) => string) {
+function execWithCustomError<ReturnType>(func: (...args: any[]) => ReturnType, showError: (error: Error) => string): ReturnType {
   try {
     return func();
   } catch (error) {
@@ -30,9 +30,9 @@ export default function verifyAndStoreOptionValue({
   requiredOptions: IState['requiredOptions'],
 }): void {
   //Verify value for correctness
-  const verifyValue = execWithCustomError(verify.bind(null, value), (error) => `Invalid value set for option "${option}", the validation of "${value}" failed: ${String(error)}`);
+  const verifyValue: boolean = execWithCustomError(verify.bind(null, value), (error) => `Invalid value set for option "${option}", the validation of "${value}" failed: ${String(error)}`);
   if (verifyValue !== true) {
-    const customMessage = execWithCustomError(message.bind(null, value), (error) => `Invalid value set for option "${option}": "${value}". Could not create custom error message: ${String(error)}`);
+    const customMessage: string | undefined = execWithCustomError(message.bind(null, value), (error) => `Invalid value set for option "${option}": "${value}". Could not create custom error message: ${String(error)}`);
     if (customMessage !== undefined && customMessage !== '') {
       throw new Error(customMessage);
     }