diff --git a/rpc/client_test.go b/rpc/client_test.go
index 04c847d0d626..51df76f7fe44 100644
--- a/rpc/client_test.go
+++ b/rpc/client_test.go
@@ -82,11 +82,15 @@ func TestClientErrorData(t *testing.T) {
 	}
 
 	// Check code.
+	// The method handler returns an error value which implements the rpc.Error
+	// interface, i.e. it has a custom error code. The server returns this error code.
+	expectedCode := testError{}.ErrorCode()
 	if e, ok := err.(Error); !ok {
 		t.Fatalf("client did not return rpc.Error, got %#v", e)
-	} else if e.ErrorCode() != (testError{}.ErrorCode()) {
-		t.Fatalf("wrong error code %d, want %d", e.ErrorCode(), testError{}.ErrorCode())
+	} else if e.ErrorCode() != expectedCode {
+		t.Fatalf("wrong error code %d, want %d", e.ErrorCode(), expectedCode)
 	}
+
 	// Check data.
 	if e, ok := err.(DataError); !ok {
 		t.Fatalf("client did not return rpc.DataError, got %#v", e)
diff --git a/rpc/errors.go b/rpc/errors.go
index 4c06a745fbd8..9a19e9fe67f5 100644
--- a/rpc/errors.go
+++ b/rpc/errors.go
@@ -54,9 +54,15 @@ var (
 	_ Error = new(invalidRequestError)
 	_ Error = new(invalidMessageError)
 	_ Error = new(invalidParamsError)
+	_ Error = new(internalServerError)
 )
 
-const defaultErrorCode = -32000
+const (
+	errcodeDefault                  = -32000
+	errcodeNotificationsUnsupported = -32001
+	errcodePanic                    = -32603
+	errcodeMarshalError             = -32603
+)
 
 type methodNotFoundError struct{ method string }
 
@@ -101,3 +107,13 @@ type invalidParamsError struct{ message string }
 func (e *invalidParamsError) ErrorCode() int { return -32602 }
 
 func (e *invalidParamsError) Error() string { return e.message }
+
+// internalServerError is used for server errors during request processing.
+type internalServerError struct {
+	code    int
+	message string
+}
+
+func (e *internalServerError) ErrorCode() int { return e.code }
+
+func (e *internalServerError) Error() string { return e.message }
diff --git a/rpc/handler.go b/rpc/handler.go
index cd95a067f3e2..22ad98149f8c 100644
--- a/rpc/handler.go
+++ b/rpc/handler.go
@@ -48,7 +48,6 @@ import (
 //    if err := op.wait(...); err != nil {
 //        h.removeRequestOp(op) // timeout, etc.
 //    }
-//
 type handler struct {
 	reg            *serviceRegistry
 	unsubscribeCb  *callback
@@ -354,7 +353,10 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 // handleSubscribe processes *_subscribe method calls.
 func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage {
 	if !h.allowSubscribe {
-		return msg.errorResponse(ErrNotificationsUnsupported)
+		return msg.errorResponse(&internalServerError{
+			code:    errcodeNotificationsUnsupported,
+			message: ErrNotificationsUnsupported.Error(),
+		})
 	}
 
 	// Subscription method name is first argument.
diff --git a/rpc/json.go b/rpc/json.go
index 6b2ac2d52a7b..1064939ff8b6 100644
--- a/rpc/json.go
+++ b/rpc/json.go
@@ -104,15 +104,14 @@ func (msg *jsonrpcMessage) errorResponse(err error) *jsonrpcMessage {
 func (msg *jsonrpcMessage) response(result interface{}) *jsonrpcMessage {
 	enc, err := json.Marshal(result)
 	if err != nil {
-		// TODO: wrap with 'internal server error'
-		return msg.errorResponse(err)
+		return msg.errorResponse(&internalServerError{errcodeMarshalError, err.Error()})
 	}
 	return &jsonrpcMessage{Version: vsn, ID: msg.ID, Result: enc}
 }
 
 func errorMessage(err error) *jsonrpcMessage {
 	msg := &jsonrpcMessage{Version: vsn, ID: null, Error: &jsonError{
-		Code:    defaultErrorCode,
+		Code:    errcodeDefault,
 		Message: err.Error(),
 	}}
 	ec, ok := err.(Error)
diff --git a/rpc/server_test.go b/rpc/server_test.go
index d09d31634bee..c9abe53e5210 100644
--- a/rpc/server_test.go
+++ b/rpc/server_test.go
@@ -45,7 +45,7 @@ func TestServerRegisterName(t *testing.T) {
 		t.Fatalf("Expected service calc to be registered")
 	}
 
-	wantCallbacks := 10
+	wantCallbacks := 12
 	if len(svc.callbacks) != wantCallbacks {
 		t.Errorf("Expected %d callbacks for service 'service', got %d", wantCallbacks, len(svc.callbacks))
 	}
diff --git a/rpc/service.go b/rpc/service.go
index bef891ea1125..cfdfba023a0a 100644
--- a/rpc/service.go
+++ b/rpc/service.go
@@ -18,7 +18,6 @@ package rpc
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"reflect"
 	"runtime"
@@ -199,7 +198,7 @@ func (c *callback) call(ctx context.Context, method string, args []reflect.Value
 			buf := make([]byte, size)
 			buf = buf[:runtime.Stack(buf, false)]
 			log.Error("RPC method " + method + " crashed: " + fmt.Sprintf("%v\n%s", err, buf))
-			errRes = errors.New("method handler crashed")
+			errRes = &internalServerError{errcodePanic, "method handler crashed"}
 		}
 	}()
 	// Run the callback.
diff --git a/rpc/testdata/internal-error.js b/rpc/testdata/internal-error.js
new file mode 100644
index 000000000000..2ba387401f24
--- /dev/null
+++ b/rpc/testdata/internal-error.js
@@ -0,0 +1,7 @@
+// These tests trigger various 'internal error' conditions.
+
+--> {"jsonrpc":"2.0","id":1,"method":"test_marshalError","params": []}
+<-- {"jsonrpc":"2.0","id":1,"error":{"code":-32603,"message":"json: error calling MarshalText for type *rpc.MarshalErrObj: marshal error"}}
+
+--> {"jsonrpc":"2.0","id":2,"method":"test_panic","params": []}
+<-- {"jsonrpc":"2.0","id":2,"error":{"code":-32603,"message":"method handler crashed"}}
diff --git a/rpc/testservice_test.go b/rpc/testservice_test.go
index 253e26328900..8454a4019222 100644
--- a/rpc/testservice_test.go
+++ b/rpc/testservice_test.go
@@ -70,6 +70,12 @@ func (testError) Error() string          { return "testError" }
 func (testError) ErrorCode() int         { return 444 }
 func (testError) ErrorData() interface{} { return "testError data" }
 
+type MarshalErrObj struct{}
+
+func (o *MarshalErrObj) MarshalText() ([]byte, error) {
+	return nil, errors.New("marshal error")
+}
+
 func (s *testService) NoArgsRets() {}
 
 func (s *testService) Echo(str string, i int, args *echoArgs) echoResult {
@@ -114,6 +120,14 @@ func (s *testService) ReturnError() error {
 	return testError{}
 }
 
+func (s *testService) MarshalError() *MarshalErrObj {
+	return &MarshalErrObj{}
+}
+
+func (s *testService) Panic() string {
+	panic("service panic")
+}
+
 func (s *testService) CallMeBack(ctx context.Context, method string, args []interface{}) (interface{}, error) {
 	c, ok := ClientFromContext(ctx)
 	if !ok {