Mistaken call to a hash table results in strange ruby irb prompt

Mistaken call to a hash table results in strange ruby irb prompt

I'm running the Ruby irb on a DOS environment.
I've defined a dictionary.
irb(main):001:0> stuff = {'name'=> 'Zed', 'age'=>36, 'height'=>6*12+2}
I've made a mistake in calling it
irb(main):004:0> puts stuff['age]
the ruby prompt changes to an apostrophe ' instead of the usual >
irb(main):006:1'
irb(main):007:1'
IRB doesn't work anymore.
What has happened here and how do I get the shell to function again
without quitting the program?