"Knock Knock - Flea" joke

Knock Knock
Who's there!
Flea!
Flea who?
Flea's a jolly good fellow! Knock Knock
Who's there!
Flea!
Flea who?
Flea forty five in a minute! Knock Knock
Who's there!
Flea!
Flea who?
Flea blind mice!