checking valid? for model using minitest for Rails 4.1.7

I am new to rails. I was trying to write unit test(mintest 5.5.1) for a model. the Database i am using is a SQL server (i use the gem activerecord-sqlserver-adapter (4.1.0)) . The tables are created fine.

Migration Script for model

class CreateRpReports < ActiveRecord::Migration
 def change
   create_table :rp_reports do |t|
     t.string :name, :null => false
     t.string :description

     t.timestamps
  end
 end
end  

Model Class

class RpReport < ActiveRecord::Base
validates :name, presence: true

 def errors
  {:name => ["can't be blank"]}
 end  
end

Unit test for RpReport Model

require 'test_helper'

describe RpReport do

def valid_params 
 { name: "John Doe", description: "John Report" }
end

####TEST 1###
it "is valid with valid params" do
 rpReport = RpReport.new valid_params    
 rpReport.must_be :valid? #must create with valid params
end

####TEST 2### 
it "is invalid without a name" do
  params = valid_params.clone
  params.delete :name
  rpReport = RpReport.new params

  rpReport.wont_be :valid? #must not be valid without name

  rpReport.errors[:name].must_be :present? #must have error for
  missing   name
 end
end 

I am getting error when i run rake test

Test Output

RpReport
test_0001_is valid with valid params                            FAIL  (0.01s)
Minitest::Assertion:         Expected #<RpReport id: nil, name: "John Doe", description: "John Report", created_at: nil, updated_at: nil> to be valid?.
    test/models/rp_report_test.rb:21:in `block (2 levels) in <top (required)>'

test_0002_is invalid without a name                            ERROR (0.00s)
NoMethodError:         NoMethodError: undefined method `add' for {:name=>["can't be blank"]}:Hash
        test/models/rp_report_test.rb:31:in `block (2 levels) in <top (required)>'
    test/models/rp_report_test.rb:31:in `block (2 levels) in <top (required)>'


Finished in 0.04500s
2 tests, 1 assertions, 1 failures, 1 errors, 0 skips

Can anyone assist in pointing out what mistake i am doing while writing the test