diff --git a/packer/plugin/server.go b/packer/plugin/server.go
index a3dbc7b24..a36f8beda 100644
--- a/packer/plugin/server.go
+++ b/packer/plugin/server.go
@@ -33,7 +33,7 @@ const MagicCookieValue = "d602bf8f470bc67ca7faa0386276bbdd4330efaf76d1a219cb4d69
 // The APIVersion is outputted along with the RPC address. The plugin
 // client validates this API version and will show an error if it doesn't
 // know how to speak it.
-const APIVersion = "2"
+const APIVersion = "3"
 
 // Server waits for a connection to this plugin and returns a Packer
 // RPC server that you can use to register components and serve them.
diff --git a/packer/rpc/builder.go b/packer/rpc/builder.go
index 986e218a2..5e3f429b5 100644
--- a/packer/rpc/builder.go
+++ b/packer/rpc/builder.go
@@ -26,7 +26,7 @@ type BuilderPrepareArgs struct {
 
 type BuilderPrepareResponse struct {
 	Warnings []string
-	Error    error
+	Error    *BasicError
 }
 
 func (b *builder) Prepare(config ...interface{}) ([]string, error) {
@@ -35,8 +35,12 @@ func (b *builder) Prepare(config ...interface{}) ([]string, error) {
 	if cerr != nil {
 		return nil, cerr
 	}
+	var err error = nil
+	if resp.Error != nil {
+		err = resp.Error
+	}
 
-	return resp.Warnings, resp.Error
+	return resp.Warnings, err
 }
 
 func (b *builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packer.Artifact, error) {
@@ -72,13 +76,9 @@ func (b *builder) Cancel() {
 
 func (b *BuilderServer) Prepare(args *BuilderPrepareArgs, reply *BuilderPrepareResponse) error {
 	warnings, err := b.builder.Prepare(args.Configs...)
-	if err != nil {
-		err = NewBasicError(err)
-	}
-
 	*reply = BuilderPrepareResponse{
 		Warnings: warnings,
-		Error:    err,
+		Error:    NewBasicError(err),
 	}
 	return nil
 }
diff --git a/packer/rpc/builder_test.go b/packer/rpc/builder_test.go
index 37e8041bb..51dbf7044 100644
--- a/packer/rpc/builder_test.go
+++ b/packer/rpc/builder_test.go
@@ -30,8 +30,9 @@ func TestBuilderPrepare(t *testing.T) {
 		t.Fatal("should be called")
 	}
 
-	if !reflect.DeepEqual(b.PrepareConfig, []interface{}{42}) {
-		t.Fatalf("bad: %#v", b.PrepareConfig)
+	expected := []interface{}{int64(42)}
+	if !reflect.DeepEqual(b.PrepareConfig, expected) {
+		t.Fatalf("bad: %#v != %#v", b.PrepareConfig, expected)
 	}
 }
 
diff --git a/packer/rpc/client.go b/packer/rpc/client.go
index 0fce79bf8..0cc8fc869 100644
--- a/packer/rpc/client.go
+++ b/packer/rpc/client.go
@@ -2,6 +2,7 @@ package rpc
 
 import (
 	"github.com/mitchellh/packer/packer"
+	"github.com/ugorji/go/codec"
 	"io"
 	"log"
 	"net/rpc"
@@ -32,9 +33,12 @@ func newClientWithMux(mux *MuxConn, streamId uint32) (*Client, error) {
 		return nil, err
 	}
 
+	var h codec.MsgpackHandle
+	clientCodec := codec.GoRpc.ClientCodec(clientConn, &h)
+
 	return &Client{
 		mux:      mux,
-		client:   rpc.NewClient(clientConn),
+		client:   rpc.NewClientWithCodec(clientCodec),
 		closeMux: false,
 	}, nil
 }
diff --git a/packer/rpc/error.go b/packer/rpc/error.go
index a5880fbac..c3ab7b1a4 100644
--- a/packer/rpc/error.go
+++ b/packer/rpc/error.go
@@ -9,6 +9,10 @@ type BasicError struct {
 }
 
 func NewBasicError(err error) *BasicError {
+	if err == nil {
+		return nil
+	}
+
 	return &BasicError{err.Error()}
 }
 
diff --git a/packer/rpc/post_processor_test.go b/packer/rpc/post_processor_test.go
index dabd8c03a..d9ea9c86f 100644
--- a/packer/rpc/post_processor_test.go
+++ b/packer/rpc/post_processor_test.go
@@ -54,7 +54,8 @@ func TestPostProcessorRPC(t *testing.T) {
 		t.Fatal("config should be called")
 	}
 
-	if !reflect.DeepEqual(p.configVal, []interface{}{42}) {
+	expected := []interface{}{int64(42)}
+	if !reflect.DeepEqual(p.configVal, expected) {
 		t.Fatalf("unknown config value: %#v", p.configVal)
 	}
 
diff --git a/packer/rpc/provisioner_test.go b/packer/rpc/provisioner_test.go
index a17e0694c..421990ffd 100644
--- a/packer/rpc/provisioner_test.go
+++ b/packer/rpc/provisioner_test.go
@@ -23,7 +23,8 @@ func TestProvisionerRPC(t *testing.T) {
 	if !p.PrepCalled {
 		t.Fatal("should be called")
 	}
-	if !reflect.DeepEqual(p.PrepConfigs, []interface{}{42}) {
+	expected := []interface{}{int64(42)}
+	if !reflect.DeepEqual(p.PrepConfigs, expected) {
 		t.Fatalf("bad: %#v", p.PrepConfigs)
 	}
 
diff --git a/packer/rpc/server.go b/packer/rpc/server.go
index 5e4b5006b..df50f48ad 100644
--- a/packer/rpc/server.go
+++ b/packer/rpc/server.go
@@ -3,6 +3,7 @@ package rpc
 import (
 	"fmt"
 	"github.com/mitchellh/packer/packer"
+	"github.com/ugorji/go/codec"
 	"io"
 	"log"
 	"net/rpc"
@@ -145,7 +146,9 @@ func (s *Server) Serve() {
 		return
 	}
 
-	s.server.ServeConn(stream)
+	var h codec.MsgpackHandle
+	rpcCodec := codec.GoRpc.ServerCodec(stream, &h)
+	s.server.ServeCodec(rpcCodec)
 }
 
 // registerComponent registers a single Packer RPC component onto