diff --git a/core/src/main/scala/caliban/schema/RootSchemaBuilder.scala b/core/src/main/scala/caliban/schema/RootSchemaBuilder.scala
index 05c950878..eaedff91d 100644
--- a/core/src/main/scala/caliban/schema/RootSchemaBuilder.scala
+++ b/core/src/main/scala/caliban/schema/RootSchemaBuilder.scala
@@ -17,10 +17,11 @@ case class RootSchemaBuilder[-R](
     )
 
   def types: List[__Type] = {
-    val empty = additionalTypes
-    (query.map(_.opType).fold(empty)(collectTypes(_)) ++
-      mutation.map(_.opType).fold(empty)(collectTypes(_)) ++
-      subscription.map(_.opType).fold(empty)(collectTypes(_)))
+    val init = additionalTypes.foldLeft(List.empty[__Type]) { case (acc, t) => collectTypes(t, acc) }
+    (init ++
+      query.map(_.opType).fold(List.empty[__Type])(collectTypes(_, init)) ++
+      mutation.map(_.opType).fold(List.empty[__Type])(collectTypes(_, init)) ++
+      subscription.map(_.opType).fold(List.empty[__Type])(collectTypes(_, init)))
       .groupBy(t => (t.name, t.kind, t.origin))
       .flatMap(_._2.headOption)
       .toList
diff --git a/core/src/test/scala/caliban/schema/SchemaSpec.scala b/core/src/test/scala/caliban/schema/SchemaSpec.scala
index 22b3e8257..ef758a1f8 100644
--- a/core/src/test/scala/caliban/schema/SchemaSpec.scala
+++ b/core/src/test/scala/caliban/schema/SchemaSpec.scala
@@ -181,6 +181,40 @@ object SchemaSpec extends DefaultRunnableSpec {
                          |  a: String!
                          |}""".stripMargin
         assertTrue(gql.render == expected)
+      },
+      test("Pass interface to withAdditionalTypes") {
+        @GQLInterface
+        sealed trait Interface
+
+        case class A(s: String) extends Interface
+        case class B(s: String) extends Interface
+
+        case class Query(a: A, b: B)
+
+        val interfaceType = Schema.gen[Any, Interface].toType_()
+
+        val gql      = graphQL(RootResolver(Query(A("a"), B("b")))).withAdditionalTypes(List(interfaceType))
+        val expected = """schema {
+                         |  query: Query
+                         |}
+                         |
+                         |interface Interface {
+                         |  s: String!
+                         |}
+                         |
+                         |type A implements Interface {
+                         |  s: String!
+                         |}
+                         |
+                         |type B implements Interface {
+                         |  s: String!
+                         |}
+                         |
+                         |type Query {
+                         |  a: A!
+                         |  b: B!
+                         |}""".stripMargin
+        assertTrue(gql.render == expected)
       }
     )
 
diff --git a/vuepress/docs/faq/README.md b/vuepress/docs/faq/README.md
index 4080e609d..9427d9558 100644
--- a/vuepress/docs/faq/README.md
+++ b/vuepress/docs/faq/README.md
@@ -107,8 +107,8 @@ case class B(s: String) extends Interface
 
 case class Query(a: A, b: B)
 
-val interfaceSchema = Schema.gen[Interface]
+val interfaceType = Schema.gen[Interface].toType_()
 
-val api = graphQL(RootResolver(Query(A("a"), B("b")))).withAdditionalTypes(List(interfaceSchema.toType_()))
+val api = graphQL(RootResolver(Query(A("a"), B("b")))).withAdditionalTypes(List(interfaceType))
 ```