diff --git a/Sources/JSONAPI/Document/Document.swift b/Sources/JSONAPI/Document/Document.swift
index 2cbf4e7..5c8160b 100644
--- a/Sources/JSONAPI/Document/Document.swift
+++ b/Sources/JSONAPI/Document/Document.swift
@@ -509,7 +509,7 @@ extension Document.SuccessDocument: Decodable, JSONAPIDocument
 
 extension Document.SuccessDocument where IncludeType == NoIncludes {
     /// Create a new Document with the given includes.
-    public func including<I: JSONAPI.Include>(_ includes: Includes<I>) -> Document<PrimaryResourceBody, MetaType, LinksType, I, APIDescription, Error> {
+    public func including<I: JSONAPI.Include>(_ includes: Includes<I>) -> Document<PrimaryResourceBody, MetaType, LinksType, I, APIDescription, Error>.SuccessDocument {
         // Note that if IncludeType is NoIncludes, then we allow anything
         // to be included, but if IncludeType already specifies a type
         // of thing to be expected then we lock that down.
@@ -531,7 +531,7 @@ extension Document.SuccessDocument where IncludeType == NoIncludes {
 extension Document.SuccessDocument where IncludeType: _Poly1 {
     /// Create a new Document adding the given includes. This does not
     /// remove existing includes; it is additive.
-    public func including(_ includes: Includes<IncludeType>) -> Document {
+    public func including(_ includes: Includes<IncludeType>) -> Document.SuccessDocument {
         // Note that if IncludeType is NoIncludes, then we allow anything
         // to be included, but if IncludeType already specifies a type
         // of thing to be expected then we lock that down.
diff --git a/Tests/JSONAPITests/Document/DocumentTests.swift b/Tests/JSONAPITests/Document/DocumentTests.swift
index 48ed1c8..bbf36e9 100644
--- a/Tests/JSONAPITests/Document/DocumentTests.swift
+++ b/Tests/JSONAPITests/Document/DocumentTests.swift
@@ -720,6 +720,7 @@ extension DocumentTests {
 
         let documentWithIncludes = document.including(Includes<Include1<Author>>(values: [.init(author)]))
 
+        XCTAssert(type(of: documentWithIncludes) == Document<NoResourceBody, NoMetadata, NoLinks, Include1<Author>, NoAPIDescription, UnknownJSONAPIError>.SuccessDocument.self)
         XCTAssertEqual(document.body.errors, documentWithIncludes.body.errors)
         XCTAssertEqual(document.body.meta, documentWithIncludes.body.meta)
         XCTAssertEqual(document.body.links, documentWithIncludes.body.links)