Unknown Method key? error in Rails 2.3.8 Unit testing

I was writing unit tests for my models for a while. After that I was
tweaking around and again continued writing unit tests.

Earlier all my unit tests were working - successfully. But now WHen I
run them, it gives me

Loaded suite unit/post_test
Started
EEEE
Finished in 0.112698 seconds.

  1) Error:
test_presence_of_body(PostTest):
NoMethodError: undefined method `key?' for #<String:0x103519a88>

  2) Error:
test_presence_of_body_and_title(PostTest):
NoMethodError: undefined method `key?' for #<String:0x1034dd420>

  3) Error:
test_presence_of_title(PostTest):
NoMethodError: undefined method `key?' for #<String:0x1034af750>

  4) Error:
test_title_minimum_width_3(PostTest):
NoMethodError: undefined method `key?' for #<String:0x103481a80>

And my test cases are

class PostTest < ActiveSupport::TestCase

  def test_presence_of_title
    post = Post.new(:body=>"Some content")
    assert !post.save,"Saved post without title"
  end

  def test_presence_of_body
    post = Post.new(:title=>"Some title")
    assert !post.save,"saved post without body"
  end

  def test_presence_of_body_and_title
    post = Post.new(:title=>"Some title",:body=>"")
    assert !post.save,"Saved Post without body"

    post = Post.new(:title => "",:body=>"Some body")
    assert !post.save,"Saved Post without title"

    post = Post.new(:title =>"",:body=>"")
    assert !post.save,"Saved Post with title and body"

  end

  def test_title_minimum_width_3
    post1 = Post.new(:title=>"a",:body=>"This will not be saved")
    assert !post1.save,"Saved post with title length less than 3"

    post2 = Post.new(:title=>"abcd",:body=>"This will be saved")
    assert post2.save,"Couldnot save a valid post record"

    post3 = Post.new(:title=>"abc",:body=>"This will be saved")
    assert post3.save,"Could not save a valid record"
  end
end

Please help

Thanks