Error creating a validation in model when i have a before_filter on my controller

I dont know why, but my application run without errors when i dont put
the line:
    "validates :age, :numericality => true" on my model

if i put that line in the model, this one error appear:

"
You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.map

Extracted source (around line #19):

16: <%= f.text_field :value %>
17: </div>
18: <div class="field">
19: <%= f.select(:owner_id,
options_from_collection_for_select(@owners, :id, :name)) %>
20: </div>
21: <div class="field">
22: <%= f.select(:reservation_type_id,
options_from_collection_for_select(@reservation_types, :id, :name))%>

"

I think that the before_filter is been skipped, but i don't know why
or how i could resolve it....

I hope that someone can help me to understand this, and thx.