is this a ruby bug or am i misunderstanding something? fails on 2.4.1p111

bug.rb

# without a default for x
def foo(x, keyword: :b)
  puts x
  puts keyword
end  

foo({x: 1})
#{:x=>1}
#b

# make x's default an empty hash
def foo(x={}, keyword: :b)
  puts x
  puts keyword
end

foo({x: 1}, keyword: :c)
# {:x=>1}
# c
foo({x: 1})
# ArgumentError: unknown keyword: x
# Why raise? why assume the has is kwargs after giving x a default of {}?