diff --git a/tomo.go b/tomo.go
index db7ae72..e3bd279 100644
--- a/tomo.go
+++ b/tomo.go
@@ -1,8 +1,10 @@
 package tomo
 
+import "sync"
 import "image"
 import "errors"
 
+var backendLock sync.Mutex
 var backend Backend
 
 // Run initializes a backend, runs the specified callback function, and runs the
@@ -17,7 +19,10 @@ func Run (callback func ()) error {
 
 	back, err := Initialize()
 	if err != nil { return err }
+	
+	backendLock.Lock()
 	backend = back
+	backendLock.Unlock()
 
 	callback()
 	return backend.Run()
@@ -32,11 +37,16 @@ func assertBackend () {
 func Stop () {
 	assertBackend()
 	backend.Stop()
+	
+	backendLock.Lock()
 	backend = nil
+	backendLock.Unlock()
 }
 
 func Do (callback func ()) {
-	backend.Do(callback)
+	backendLock.Lock()
+	if backend != nil { backend.Do(callback) }
+	backendLock.Unlock()
 }
 
 func NewWindow (bounds image.Rectangle) (MainWindow, error) {