diff --git a/mw_js_plugin.go b/mw_js_plugin.go
index e14079f8c62f..f9d13d227666 100644
--- a/mw_js_plugin.go
+++ b/mw_js_plugin.go
@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"strings"
 	"time"
+	"sync"
 
 	"github.com/robertkrimen/otto"
 	_ "github.com/robertkrimen/otto/underscore"
@@ -280,6 +281,7 @@ func mapStrsToIfaces(m map[string]string) map[string]interface{} {
 // --- Utility functions during startup to ensure a sane VM is present for each API Def ----
 
 type JSVM struct {
+	sync.Mutex
 	Spec    *APISpec
 	VM      *otto.Otto
 	Timeout time.Duration
@@ -290,6 +292,9 @@ type JSVM struct {
 // Init creates the JSVM with the core library and sets up a default
 // timeout.
 func (j *JSVM) Init(spec *APISpec) {
+	j.Lock()
+	defer j.Unlock()
+
 	vm := otto.New()
 
 	// Init TykJS namespace, constructors etc.