diff --git a/README.md b/README.md
index dd6bce7..a7101fa 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ commands to the handler.
 
 ## Usage
 
-1. Compile your `.yarn` file. You can probably get the compiled output from a  
+1. Compile your `.yarn` file. You can probably get the compiled output from a
    Unity project, or you can compile without using Unity with a tool like the
    [Yarn Spinner Console](https://github.com/YarnSpinnerTool/YarnSpinner-Console):
 
@@ -75,7 +75,7 @@ commands to the handler.
        return choice, nil
    }
 
-   // ... and also the other methods. 
+   // ... and also the other methods.
    // Alternatively you can embed yarn.FakeDialogueHandler in your handler.
    ```
 
@@ -85,9 +85,9 @@ commands to the handler.
 
    ```go
    package main
-   
+
    import "github.com/DrJosh9000/yarn"
-   
+
    func main() {
        // Load the files (error handling omitted for brevity):
        program, stringTable, _ := yarn.LoadFiles("Example.yarn.yarnc", "Example.yarn.csv", "en-AU")
@@ -99,7 +99,7 @@ commands to the handler.
        vm := &yarn.VirtualMachine{
            Program: program,
            Handler: myHandler,
-           Vars: make(yarn.MapVariableStorage), // or your own VariableStorage implementation
+           Vars:    yarn.NewMapVariableStorage(), // or your own VariableStorage implementation
            FuncMap: yarn.FuncMap{ // this is optional
                "last_value": func(x ...interface{}) interface{} {
                    return x[len(x)-1]
@@ -168,7 +168,7 @@ func (m *MyHandler) setWaiting(w bool) {
 func (m *MyHandler) Line(line yarn.Line) error {
     text, _ := m.stringTable.Render(line)
     m.dialogueDisplay.Show(text)
-    
+
     // Go into waiting-for-player-input state
     m.setWaiting(true)
 
diff --git a/cmd/yarnrunner.go b/cmd/yarnrunner.go
index 522c312..4635e8f 100644
--- a/cmd/yarnrunner.go
+++ b/cmd/yarnrunner.go
@@ -44,7 +44,7 @@ func main() {
 	langCode := flag.String("lang", "en-AU", "Language tag (BCP 47)")
 	flag.Parse()
 
-	program, stringTable, err := yarn.LoadFiles(*yarncFilename, *csvFilename, *langCode)
+	program, stringTable, err := yarn.LoadFiles(*yarncFilename, *langCode)
 	if err != nil {
 		log.Fatalf("Loading files: %v", err)
 	}
@@ -54,7 +54,7 @@ func main() {
 		Handler: &dialogueHandler{
 			stringTable: stringTable,
 		},
-		Vars: make(yarn.MapVariableStorage),
+		Vars: yarn.NewMapVariableStorage(),
 	}
 	if err := vm.Run(*startNode); err != nil {
 		log.Printf("Yarn VM error: %v", err)
diff --git a/vars.go b/vars.go
index 4bec47c..b579f59 100644
--- a/vars.go
+++ b/vars.go
@@ -14,30 +14,48 @@
 
 package yarn
 
+import "sync"
+
 // VariableStorage stores values of any kind.
 type VariableStorage interface {
 	Clear()
-	GetValue(name string) (value interface{}, ok bool)
-	SetValue(name string, value interface{})
+	GetValue(name string) (value any, ok bool)
+	SetValue(name string, value any)
 }
 
 // MapVariableStorage implements VariableStorage, in memory, using a map.
-type MapVariableStorage map[string]interface{}
+type MapVariableStorage struct {
+	mu sync.RWMutex
+	m  map[string]any
+}
+
+// NewMapVariableStorage creates a new empty MapVariableStorage.
+func NewMapVariableStorage() *MapVariableStorage {
+	return &MapVariableStorage{
+		m: make(map[string]any),
+	}
+}
 
 // Clear empties the storage of all values.
-func (m MapVariableStorage) Clear() {
-	for name := range m {
-		delete(m, name)
+func (m *MapVariableStorage) Clear() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for name := range m.m {
+		delete(m.m, name)
 	}
 }
 
 // GetValue fetches a value from the map, returning (nil, false) if not present.
-func (m MapVariableStorage) GetValue(name string) (value interface{}, found bool) {
-	value, found = m[name]
+func (m *MapVariableStorage) GetValue(name string) (value any, found bool) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	value, found = m.m[name]
 	return value, found
 }
 
 // SetValue sets a value in the map.
-func (m MapVariableStorage) SetValue(name string, value interface{}) {
-	m[name] = value
+func (m *MapVariableStorage) SetValue(name string, value any) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.m[name] = value
 }
diff --git a/vm_test.go b/vm_test.go
index 19bb632..54648d2 100644
--- a/vm_test.go
+++ b/vm_test.go
@@ -47,7 +47,7 @@ func TestAllTestPlans(t *testing.T) {
 			vm := &VirtualMachine{
 				Program: prog,
 				Handler: testplan,
-				Vars:    make(MapVariableStorage),
+				Vars:    NewMapVariableStorage(),
 				FuncMap: FuncMap{
 					// Used by various
 					"assert": func(x interface{}) error {