You don't change the value of a , you create a new object and make a point to its value. Mutable objects cannot be hashed. See this answer. To solve your problem, you should use immutable objects as keys in your dictionary.

Are hashes immutable?

Since it is immutable, all methods which you might expect to "modify" a Immutable::Hash actually return a new hash and leave the existing one unchanged.

What does it mean if a variable is mutable?

Mutable is a type of variable that can be changed. A mutable object is an object whose state can be modified after it is created. ... Immutables are the objects whose state cannot be changed once the object is created.