Ruby Extra Credit Help

For the extra credit of "Build a Bank Account Class" in "Ruby Objects and Classes", we are asked to rewrite the balance method of the BankAccount class we had written in the course to use the inject method. inject takes a block, which in turn takes 2 parameters: an accumulator variable and the object it is currently operating on, like this:

(5..10).inject do |sum, n|
    sum + n
end
#In the end, sum will be 45, and inject will return that

The balance method we wrote during the course looks like this:

def balance
    value = 0
    @transactions.each do |transaction|
        value += transaction[:amount]
    end
    return value
end

@transactions is an array of hashes, which might look something like this:

{description: "Paycheck", amount: 100}

Each hash in the @transactions array has a description, which is supposed to list the purpose of the transaction, and it also holds the amount of that transaction. The goal of the balance method is to iterate over all the hashes in the @transactions array and add together all the amounts. Here's my attempt at doing it with the inject method, like the Extra Credit asks:

def balance
    @transactions.inject do |sum, transaction|
        sum[:amount] + transaction[:amount]
    end
end

However, this returns the following error & stack trace:

bank.rb:19:in `[]': no implicit conversion of Symbol into Integer (TypeError)                    
        from bank.rb:19:in `block in balance'                                                    
        from bank.rb:18:in `each'                                                                
        from bank.rb:18:in `inject'                                                              
        from bank.rb:18:in `balance'                                                             
        from bank.rb:30:in `print_register'                                                      
        from bank.rb:36:in `<main>'

How can I adapt the balance method to properly use inject?