diff --git a/CHANGELOG.md b/CHANGELOG.md
index 304707bbe..ba20ed32f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,10 @@ Features:
 - [#1340](https://github.com/rails-api/active_model_serializers/pull/1340) Add support for resource-level meta. (@beauby)
 
 Fixes:
+- [#1615](https://github.com/rails-api/active_model_serializers/pull/1615) Allow plural polymorphic relationship types when validating
+  presence. (@dpdawson)
+- [#1615](https://github.com/rails-api/active_model_serializers/pull/1615) Capitalize polymorphic association types to prevent error
+  when ActiveRecord constantizes them when validating presence. (@dpdawson)
 - [#1478](https://github.com/rails-api/active_model_serializers/pull/1478) Cache store will now be correctly set when serializers are
   loaded *before* Rails initializes. (@bf4)
 - [#1570](https://github.com/rails-api/active_model_serializers/pull/1570) Fixed pagination issue with last page size. (@bmorrall)
diff --git a/docs/general/deserialization.md b/docs/general/deserialization.md
index 56dda833c..543d46c88 100644
--- a/docs/general/deserialization.md
+++ b/docs/general/deserialization.md
@@ -91,7 +91,7 @@ ActiveModelSerializers::Deserialization
 #   title: 'Title 1',
 #   published_at: '2015-12-20',
 #   author_id: '2',
-#   author_type: 'user'
+#   author_type: 'User'
 # }
 ```
 
diff --git a/lib/active_model_serializers/adapter/json_api/deserialization.rb b/lib/active_model_serializers/adapter/json_api/deserialization.rb
index f52443552..ec2982f81 100644
--- a/lib/active_model_serializers/adapter/json_api/deserialization.rb
+++ b/lib/active_model_serializers/adapter/json_api/deserialization.rb
@@ -69,7 +69,7 @@ module Deserialization
         #     #   title: 'Title 1',
         #     #   published_at: '2015-12-20',
         #     #   author_id: '2',
-        #     #   author_type: 'people'
+        #     #   author_type: 'People'
         #     # }
         #
         def parse!(document, options = {})
@@ -188,7 +188,7 @@ def parse_relationship(assoc_name, assoc_data, options)
             end
 
           polymorphic = (options[:polymorphic] || []).include?(assoc_name.to_sym)
-          hash["#{prefix_key}_type".to_sym] = assoc_data['type'] if polymorphic
+          hash["#{prefix_key}_type".to_sym] = assoc_data['type'].singularize.capitalize if polymorphic
 
           hash
         end
diff --git a/test/active_record_test.rb b/test/active_record_test.rb
index 5bb941a46..72a644674 100644
--- a/test/active_record_test.rb
+++ b/test/active_record_test.rb
@@ -5,5 +5,38 @@ class ActiveRecordTest < ActiveSupport::TestCase
 
   def setup
     @resource = ARModels::Post.new
+    @image = Image.create
+  end
+
+  def test_active_model_record_with_validated_polymorphic_relationship_creation
+    picture = Picture.create!(picture_params)
+
+    assert_equal(@image, picture.imageable)
+  end
+
+  private
+
+  def picture_params
+    params = ActionController::Parameters.new({
+      data: {
+        attributes: {
+          title: 'Picture Title'
+        },
+        relationships: {
+          imageable: {
+            data: {
+              type: 'images',
+              id: @image.id
+            }
+          },
+        },
+        type: 'pictures'
+      }
+    })
+
+    ActiveModelSerializers::Deserialization.jsonapi_parse!(
+      params.to_unsafe_h,
+      polymorphic: [:imageable]
+    )
   end
 end
diff --git a/test/adapter/json_api/parse_test.rb b/test/adapter/json_api/parse_test.rb
index bee79c8c1..777abf3e1 100644
--- a/test/adapter/json_api/parse_test.rb
+++ b/test/adapter/json_api/parse_test.rb
@@ -125,7 +125,7 @@ def test_polymorphic
               src: 'http://example.com/images/productivity.png',
               author_id: nil,
               photographer_id: '9',
-              photographer_type: 'people',
+              photographer_type: 'Person',
               comment_ids: %w(1 2)
             }
             assert_equal(expected, parsed_hash)
diff --git a/test/fixtures/active_record.rb b/test/fixtures/active_record.rb
index 3f0b2dc09..9407dde2f 100644
--- a/test/fixtures/active_record.rb
+++ b/test/fixtures/active_record.rb
@@ -26,8 +26,10 @@
   end
   create_table :pictures, force: true do |t|
     t.string :title
-    t.string :imageable_type
-    t.string :imageable_id
+    t.references :imageable, polymorphic: true
+    t.timestamp null: false
+  end
+  create_table :images, force: true do |t|
     t.timestamp null: false
   end
 end
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index c7fb831c8..91a3ee293 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -78,6 +78,12 @@ class Employee < ActiveRecord::Base
 
 class Picture < ActiveRecord::Base
   belongs_to :imageable, polymorphic: true
+
+  validates :imageable, presence: true
+end
+
+class Image < ActiveRecord::Base
+  has_many :pictures, as: :imageable
 end
 
 module Spam; end