rake aborted when $ rake fake:data

[imagirl92]

I'm trying to generate fake data with "rake fake:data" but I get this :

Populating database with fake data

rake aborted!
NameError: uninitialized constant Faker
/Users/admin/routes_controller_exercise/lib/tasks/fake_data.rake:12:in create_user' /Users/admin/routes_controller_exercise/lib/tasks/fake_data.rake:26:inblock (3 levels) in <top (required)>'
/Users/admin/routes_controller_exercise/lib/tasks/fake_data.rake:25:in times' /Users/admin/routes_controller_exercise/lib/tasks/fake_data.rake:25:inblock (2 levels) in <top (required)>'
Tasks: TOP => fake:data
(See full trace by running task with --trace)

I have no idea what to do and I'm a beginner. Thanks a lot!