What's the difference between "Hash.new(0)" and "{}"

Question

I was getting an unexpected (to me) behavior in my code so I tried to isolate the problem in the REPL. However these constructors both appear to have the same result (an empty hash):

irb> a = {}
# => {}

irb> b = Hash.new(0)
# => {}

When I pass {} into a reduce function I get a NoMethodError, though. What is the difference between these two constructors?

irb> arr = "count the occurance of each of the words".scan(/\w+/)
# => ["count", "the", "occurance", "of", "each", "of", "the", "words"]

irb> x = arr.reduce(Hash.new(0)) { |hsh, word| hsh[word] += 1; hsh }
# => {"count"=>1, "the"=>2, "occurance"=>1, "of"=>2, "each"=>1, "words"=>1}

irb> x = arr.reduce({}) { |hsh, word| hsh[word] += 1; hsh }
# NoMethodError: undefined method `+' for nil:NilClass

Answer 1

Hash.new(0) sets default value for any key to 0, while {} sets nil

h1 = Hash.new(0)
h1.default  # => 0
h1[:a] += 1 # => 1
h2 = {}
h2.default  # => nil
h2[:a] += 1 # => NoMethodError: undefined method `+' for nil:NilClass