diff --git a/app/controllers/registrar/deposits_controller.rb b/app/controllers/registrar/deposits_controller.rb
index ec6d13977..818e38c6d 100644
--- a/app/controllers/registrar/deposits_controller.rb
+++ b/app/controllers/registrar/deposits_controller.rb
@@ -10,12 +10,12 @@ class Registrar
       @deposit = Deposit.new(deposit_params.merge(registrar: current_user.registrar))
       @invoice = @deposit.issue_prepayment_invoice
 
-      if @invoice&.persisted?
+      if @invoice
         flash[:notice] = t(:please_pay_the_following_invoice)
         redirect_to [:registrar, @invoice]
       else
-        flash.now[:alert] = t(:failed_to_create_record)
-        render 'new'
+        flash[:alert] = @deposit.errors.full_messages.join(', ')
+        redirect_to new_registrar_deposit_path
       end
     end
 
diff --git a/app/models/deposit.rb b/app/models/deposit.rb
index 23045196a..2eff26bcc 100644
--- a/app/models/deposit.rb
+++ b/app/models/deposit.rb
@@ -4,13 +4,17 @@ class Deposit
   extend ActiveModel::Naming
   include DisableHtml5Validation
 
-  attr_accessor :amount, :description, :registrar, :registrar_id
+  attr_accessor :description, :registrar, :registrar_id
+  attr_writer :amount
 
   validates :amount, :registrar, presence: true
   validate :validate_amount
+
   def validate_amount
-    return if BigDecimal.new(amount) >= Setting.minimum_deposit
-    errors.add(:amount, I18n.t(:is_too_small_minimum_deposit_is, amount: Setting.minimum_deposit, currency: 'EUR'))
+    minimum_allowed_amount = [0.01, Setting.minimum_deposit].max
+    return if amount >= minimum_allowed_amount
+    errors.add(:amount, I18n.t(:is_too_small_minimum_deposit_is, amount: minimum_allowed_amount,
+                                                                 currency: 'EUR'))
   end
 
   def initialize(attributes = {})
@@ -24,10 +28,12 @@ class Deposit
   end
 
   def amount
-    BigDecimal.new(@amount.to_s.sub(/,/, '.'))
+    return BigDecimal('0.0') if @amount.blank?
+    BigDecimal(@amount.to_s.tr(',', '.'), 10)
   end
 
   def issue_prepayment_invoice
-    valid? && registrar.issue_prepayment_invoice(amount, description)
+    return unless valid?
+    registrar.issue_prepayment_invoice(amount, description)
   end
 end
diff --git a/spec/models/deposit_spec.rb b/spec/models/deposit_spec.rb
deleted file mode 100644
index ff77dbd98..000000000
--- a/spec/models/deposit_spec.rb
+++ /dev/null
@@ -1,39 +0,0 @@
-require 'rails_helper'
-
-describe Deposit do
-  context 'with invalid attribute' do
-    before :all do
-      @deposit = Deposit.new
-    end
-
-    it 'should not be valid' do
-      @deposit.valid?
-      @deposit.errors.full_messages.should match_array([
-        "Registrar is missing"
-      ])
-    end
-
-    it 'should have 0 amount' do
-      @deposit.amount.should == 0
-    end
-
-    it 'should not be presisted' do
-      @deposit.persisted?.should == false
-    end
-
-    it 'should replace comma with point for 0' do
-      @deposit.amount = '0,0'
-      @deposit.amount.should == 0.0
-    end
-
-    it 'should replace comma with points' do
-      @deposit.amount = '10,11'
-      @deposit.amount.should == 10.11
-    end
-
-    it 'should work with float as well' do
-      @deposit.amount = 0.123
-      @deposit.amount.should == 0.123
-    end
-  end
-end
diff --git a/test/models/deposit_test.rb b/test/models/deposit_test.rb
new file mode 100644
index 000000000..b7510b960
--- /dev/null
+++ b/test/models/deposit_test.rb
@@ -0,0 +1,59 @@
+require 'test_helper'
+
+class DepositTest < ActiveSupport::TestCase
+  def setup
+    super
+
+    @deposit = Deposit.new(registrar: registrars(:bestnames))
+    @minimum_deposit = Setting.minimum_deposit
+    Setting.minimum_deposit = 1.00
+  end
+
+  def teardown
+    super
+
+    Setting.minimum_deposit = @minimum_deposit
+  end
+
+  def test_validate_amount_cannot_be_lower_than_0_01
+    Setting.minimum_deposit = 0.0
+    @deposit.amount = -10
+    refute(@deposit.valid?)
+    assert(@deposit.errors.full_messages.include?("Amount is too small. Minimum deposit is 0.01 EUR"))
+  end
+
+  def test_validate_amount_cannot_be_lower_than_minimum_deposit
+    @deposit.amount = 0.10
+    refute(@deposit.valid?)
+
+    assert(@deposit.errors.full_messages.include?("Amount is too small. Minimum deposit is 1.0 EUR"))
+  end
+
+  def test_registrar_must_be_set
+    deposit = Deposit.new(amount: 120)
+    refute(deposit.valid?)
+
+    assert(deposit.errors.full_messages.include?("Registrar is missing"))
+  end
+
+  def test_amount_is_converted_from_string
+    @deposit.amount = "12.00"
+    assert_equal(BigDecimal.new("12.00"), @deposit.amount)
+
+    @deposit.amount = "12,11"
+    assert_equal(BigDecimal.new("12.11"), @deposit.amount)
+  end
+
+  def test_amount_is_converted_from_float
+    @deposit.amount = 12.0044
+    assert_equal(BigDecimal.new("12.0044"), @deposit.amount)
+
+    @deposit.amount = 12.0144
+    assert_equal(BigDecimal.new("12.0144"), @deposit.amount)
+  end
+
+  def test_amount_is_converted_from_nil
+    @deposit.amount = nil
+    assert_equal(BigDecimal.new("0.00"), @deposit.amount)
+  end
+end
diff --git a/test/system/registrar_area/invoices/new_test.rb b/test/system/registrar_area/invoices/new_test.rb
index b9b6b6db4..a5a72fbe8 100644
--- a/test/system/registrar_area/invoices/new_test.rb
+++ b/test/system/registrar_area/invoices/new_test.rb
@@ -29,11 +29,10 @@ class NewInvoiceTest < ApplicationSystemTestCase
     assert_text 'Pay invoice'
   end
 
-  # This test case should fail once issue #651 gets fixed
-  def test_create_new_invoice_with_amount_0_goes_through
+  def test_create_new_invoice_with_comma_in_number
     visit registrar_invoices_path
     click_link_or_button 'Add deposit'
-    fill_in 'Amount', with: '0.00'
+    fill_in 'Amount', with: '200,00'
     fill_in 'Description', with: 'My first invoice'
 
     assert_difference 'Invoice.count', 1 do
@@ -42,7 +41,33 @@ class NewInvoiceTest < ApplicationSystemTestCase
 
     assert_text 'Please pay the following invoice'
     assert_text 'Invoice no. 131050'
-    assert_text 'Subtotal 0,00 €'
+    assert_text 'Subtotal 200,00 €'
     assert_text 'Pay invoice'
   end
+
+  def test_create_new_invoice_fails_when_amount_is_0
+    visit registrar_invoices_path
+    click_link_or_button 'Add deposit'
+    fill_in 'Amount', with: '0.00'
+    fill_in 'Description', with: 'My first invoice'
+
+    assert_no_difference 'Invoice.count' do
+      click_link_or_button 'Add'
+    end
+
+    assert_text 'Amount is too small. Minimum deposit is 0.01 EUR'
+  end
+
+  def test_create_new_invoice_fails_when_amount_is_negative
+    visit registrar_invoices_path
+    click_link_or_button 'Add deposit'
+    fill_in 'Amount', with: '-120.00'
+    fill_in 'Description', with: 'My first invoice'
+
+    assert_no_difference 'Invoice.count' do
+      click_link_or_button 'Add'
+    end
+
+    assert_text 'Amount is too small. Minimum deposit is 0.01 EUR'
+  end
 end