Union types in TypeScript allow a variable to take on multiple possible types, but the compiler needs clear type guards or assertions to narrow down which type is currently in use. Without these, TypeScript might throw errors or fail to narrow the type correctly. To fix this, use type guards like `typeof` for primitive types or `instanceof` for objects to tell TypeScript exactly what type it’s dealing with. If your union involves more complex types, you can use custom type predicates or `in` checks for object properties to help TypeScript infer the correct type. With proper type narrowing, your union types will behave predictably and type-safely, ensuring that TypeScript can infer the right type in any given context.