diff --git a/Sources/JSONAPI/Document/ResourceBody.swift b/Sources/JSONAPI/Document/ResourceBody.swift
index bdcb55d..5aad51e 100644
--- a/Sources/JSONAPI/Document/ResourceBody.swift
+++ b/Sources/JSONAPI/Document/ResourceBody.swift
@@ -5,10 +5,12 @@
 //  Created by Mathew Polzin on 11/10/18.
 //
 
+public protocol PrimaryResource: Equatable, Codable {}
+
 public protocol ResourceBody: Codable, Equatable {
 }
 
-public struct SingleResourceBody<Entity: JSONAPI.EntityType>: ResourceBody {
+public struct SingleResourceBody<Entity: JSONAPI.PrimaryResource>: ResourceBody {
 	public let value: Entity?
 
 	public init(entity: Entity?) {
diff --git a/Sources/JSONAPI/Resource/Entity.swift b/Sources/JSONAPI/Resource/Entity.swift
index 19954eb..4927f5f 100644
--- a/Sources/JSONAPI/Resource/Entity.swift
+++ b/Sources/JSONAPI/Resource/Entity.swift
@@ -37,7 +37,7 @@ public protocol EntityDescription {
 /// EntityType is the protocol that Entity conforms to. This
 /// protocol lets other types accept any Entity as a generic
 /// specialization.
-public protocol EntityType: Codable, Equatable {
+public protocol EntityType: PrimaryResource {
 	associatedtype Description: EntityDescription
 	associatedtype Identifier: Equatable & Codable
 }
diff --git a/Sources/JSONAPI/Resource/Poly.swift b/Sources/JSONAPI/Resource/Poly.swift
index 3b534cf..bc3f4ba 100644
--- a/Sources/JSONAPI/Resource/Poly.swift
+++ b/Sources/JSONAPI/Resource/Poly.swift
@@ -15,7 +15,7 @@ import Result
 /// disparate types under one roof for
 /// the purposes of JSON API compliant
 /// encoding or decoding.
-public protocol Poly: Codable, Equatable {}
+public protocol Poly: PrimaryResource {}
 
 // MARK: - Generic Decoding
 
diff --git a/Tests/JSONAPITests/Document/DocumentTests.swift b/Tests/JSONAPITests/Document/DocumentTests.swift
index e2bee72..00ab40e 100644
--- a/Tests/JSONAPITests/Document/DocumentTests.swift
+++ b/Tests/JSONAPITests/Document/DocumentTests.swift
@@ -175,6 +175,14 @@ class DocumentTests: XCTestCase {
 		test_DecodeEncodeEquality(type: JSONAPIDocument<SingleResourceBody<Article>, TestPageMetadata, Include1<Author>, BasicJSONAPIError>.self,
 								  data: single_document_some_includes_with_metadata)
 	}
+
+	func test_singleDocument_PolyPrimaryResource() {
+		let article = Article(id: Id(rawValue: "1"), relationships: .init(author: ToOneRelationship(id: Id(rawValue: "33"))))
+		let document = decoded(type: JSONAPIDocument<SingleResourceBody<Poly2<Article, Author>>, NoMetadata, NoIncludes, BasicJSONAPIError>.self, data: single_document_no_includes)
+
+		XCTAssertEqual(document.body.data?.primary.value?[Article.self], article)
+		XCTAssertNil(document.body.data?.primary.value?[Author.self])
+	}
 	
 	func test_manyDocumentNoIncludes() {
 		let document = decoded(type: JSONAPIDocument<ManyResourceBody<Article>, NoMetadata, NoIncludes, BasicJSONAPIError>.self,
@@ -215,7 +223,6 @@ class DocumentTests: XCTestCase {
 		test_DecodeEncodeEquality(type: JSONAPIDocument<ManyResourceBody<Article>, NoMetadata, Include1<Author>, BasicJSONAPIError>.self,
 							   data: many_document_some_includes)
 	}
-
 }
 
 // MARK: - Test Types