diff --git a/plugin/federation/federation.go b/plugin/federation/federation.go
index 40e812a7bb0..7d9abc9774c 100644
--- a/plugin/federation/federation.go
+++ b/plugin/federation/federation.go
@@ -95,11 +95,13 @@ func (f *federation) InjectSourceLate(schema *ast.Schema) *ast.Source {
 		}
 		entities += e.Name
 
-		resolverArgs := ""
-		for _, field := range e.KeyFields {
-			resolverArgs += fmt.Sprintf("%s: %s,", field.Field.Name, field.Field.Type.String())
+		if e.ResolverName != "" {
+			resolverArgs := ""
+			for _, field := range e.KeyFields {
+				resolverArgs += fmt.Sprintf("%s: %s,", field.Field.Name, field.Field.Type.String())
+			}
+			resolvers += fmt.Sprintf("\t%s(%s): %s!\n", e.ResolverName, resolverArgs, e.Def.Name)
 		}
-		resolvers += fmt.Sprintf("\t%s(%s): %s!\n", e.ResolverName, resolverArgs, e.Def.Name)
 
 	}
 
@@ -109,18 +111,24 @@ func (f *federation) InjectSourceLate(schema *ast.Schema) *ast.Source {
 		return nil
 	}
 
+	// resolvers can be empty if a service defines only "empty
+	// extend" types.  This should be rare.
+	if resolvers != "" {
+		resolvers = `
+# fake type to build resolver interfaces for users to implement
+type Entity {
+	` + resolvers + `
+}
+`
+	}
+
 	return &ast.Source{
 		Name:    "federation/entity.graphql",
 		BuiltIn: true,
 		Input: `
 # a union of all types that use the @key directive
 union _Entity = ` + entities + `
-
-# fake type to build resolver interfaces for users to implement
-type Entity {
-	` + resolvers + `
-}
-
+` + resolvers + `
 type _Service {
   sdl: String
 }
@@ -162,9 +170,20 @@ type RequireField struct {
 	TypeReference *config.TypeReference // The Go representation of that field type
 }
 
+func (e *Entity) allFieldsAreExternal() bool {
+	for _, field := range e.Def.Fields {
+		if field.Directives.ForName("external") == nil {
+			return false
+		}
+	}
+	return true
+}
+
 func (f *federation) GenerateCode(data *codegen.Data) error {
 	if len(f.Entities) > 0 {
-		data.Objects.ByName("Entity").Root = true
+		if data.Objects.ByName("Entity") != nil {
+			data.Objects.ByName("Entity").Root = true
+		}
 		for _, e := range f.Entities {
 			obj := data.Objects.ByName(e.Def.Name)
 			for _, field := range obj.Fields {
@@ -251,13 +270,36 @@ func (f *federation) setEntities(schema *ast.Schema) {
 
 				}
 
-				f.Entities = append(f.Entities, &Entity{
+				e := &Entity{
 					Name:         schemaType.Name,
 					KeyFields:    keyFields,
 					Def:          schemaType,
 					ResolverName: resolverName,
 					Requires:     requires,
-				})
+				}
+				// If our schema has a field with a type defined in
+				// another service, then we need to define an "empty
+				// extend" of that type in this service, so this service
+				// knows what the type is like.  But the graphql-server
+				// will never ask us to actually resolve this "empty
+				// extend", so we don't require a resolver function for
+				// it.  (Well, it will never ask in practice; it's
+				// unclear whether the spec guarantees this.  See
+				// https://github.com/apollographql/apollo-server/issues/3852
+				// ).  Example:
+				//    type MyType {
+				//       myvar: TypeDefinedInOtherService
+				//    }
+				//    // Federation needs this type, but
+				//    // it doesn't need a resolver for it!
+				//    extend TypeDefinedInOtherService @key(fields: "id") {
+				//       id: ID @external
+				//    }
+				if e.allFieldsAreExternal() {
+					e.ResolverName = ""
+				}
+
+				f.Entities = append(f.Entities, e)
 			}
 		}
 	}
diff --git a/plugin/federation/federation.gotpl b/plugin/federation/federation.gotpl
index 2661495a388..96c25e85723 100644
--- a/plugin/federation/federation.gotpl
+++ b/plugin/federation/federation.gotpl
@@ -34,29 +34,31 @@ func (ec *executionContext) __resolve_entities(ctx context.Context, representati
 		}
 		switch typeName {
 		{{ range .Entities }}
-		case "{{.Def.Name}}":
-			{{ range $i, $keyField := .KeyFields -}}
-				id{{$i}}, err := ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{$keyField.Field.Name}}"])
-				if err != nil {
-					return nil, errors.New(fmt.Sprintf("Field %s undefined in schema.", "{{$keyField.Field.Name}}"))
-				}
-			{{end}}
-
-			entity, err := ec.resolvers.Entity().{{.ResolverName | go}}(ctx,
-				{{ range $i, $_ := .KeyFields -}} id{{$i}}, {{end}})
-			if err != nil {
-				return nil, err
-			}
-
-			{{ range .Requires }}
-				{{ range .Fields}}
-					entity.{{.NameGo}}, err = ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{.Name}}"])
+			{{ if .ResolverName }}
+			case "{{.Def.Name}}":
+				{{ range $i, $keyField := .KeyFields -}}
+					id{{$i}}, err := ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{$keyField.Field.Name}}"])
 					if err != nil {
-						return nil, err
+						return nil, errors.New(fmt.Sprintf("Field %s undefined in schema.", "{{$keyField.Field.Name}}"))
 					}
+				{{end}}
+	
+				entity, err := ec.resolvers.Entity().{{.ResolverName | go}}(ctx,
+					{{ range $i, $_ := .KeyFields -}} id{{$i}}, {{end}})
+				if err != nil {
+					return nil, err
+				}
+	
+				{{ range .Requires }}
+					{{ range .Fields}}
+						entity.{{.NameGo}}, err = ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{.Name}}"])
+						if err != nil {
+							return nil, err
+						}
+					{{ end }}
 				{{ end }}
+				list = append(list, entity)
 			{{ end }}
-			list = append(list, entity)
 		{{ end }}
 		default:
 			return nil, errors.New("unknown type: "+typeName)
diff --git a/plugin/resolvergen/testdata/followschema/out/resolver.go b/plugin/resolvergen/testdata/followschema/out/resolver.go
index 61296288110..fbe00ecff89 100644
--- a/plugin/resolvergen/testdata/followschema/out/resolver.go
+++ b/plugin/resolvergen/testdata/followschema/out/resolver.go
@@ -1,6 +1,7 @@
+package customresolver
+
 // This file will not be regenerated automatically.
 //
 // It serves as dependency injection for your app, add any dependencies you require here.
-package customresolver
 
 type CustomResolverType struct{}
diff --git a/plugin/resolvergen/testdata/singlefile/out/resolver.go b/plugin/resolvergen/testdata/singlefile/out/resolver.go
index 99a46892ae4..54d778636ae 100644
--- a/plugin/resolvergen/testdata/singlefile/out/resolver.go
+++ b/plugin/resolvergen/testdata/singlefile/out/resolver.go
@@ -1,6 +1,7 @@
-// THIS CODE IS A STARTING POINT ONLY. IT WILL NOT BE UPDATED WITH SCHEMA CHANGES.
 package customresolver
 
+// THIS CODE IS A STARTING POINT ONLY. IT WILL NOT BE UPDATED WITH SCHEMA CHANGES.
+
 import (
 	"context"
 )
@@ -10,11 +11,15 @@ type CustomResolverType struct{}
 func (r *queryCustomResolverType) Resolver(ctx context.Context) (*Resolver, error) {
 	panic("not implemented")
 }
+
 func (r *resolverCustomResolverType) Name(ctx context.Context, obj *Resolver) (string, error) {
 	panic("not implemented")
 }
 
-func (r *CustomResolverType) Query() QueryResolver       { return &queryCustomResolverType{r} }
+// Query returns QueryResolver implementation.
+func (r *CustomResolverType) Query() QueryResolver { return &queryCustomResolverType{r} }
+
+// Resolver returns ResolverResolver implementation.
 func (r *CustomResolverType) Resolver() ResolverResolver { return &resolverCustomResolverType{r} }
 
 type queryCustomResolverType struct{ *CustomResolverType }