diff --git a/docs/Language-Definition.md b/docs/Language-Definition.md
index 440c268dd..447726c29 100644
--- a/docs/Language-Definition.md
+++ b/docs/Language-Definition.md
@@ -148,7 +148,7 @@ user.Age > 30 ? "mature" : "immature"
 
 ## Builtin functions
 
-* `len` (length of array or string)
+* `len` (length of array, map or string)
 * `all` (will return `true` if all element satisfies the predicate)
 * `none` (will return `true` if all element does NOT satisfies the predicate)
 * `any` (will return `true` if any element satisfies the predicate)
diff --git a/expr_test.go b/expr_test.go
index 3f04d6c9a..7a9fcb882 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -684,6 +684,18 @@ func TestExpr(t *testing.T) {
 			`len(["hello", "world"])`,
 			2,
 		},
+		{
+			`len("hello, world")`,
+			12,
+		},
+		{
+			`len(Array)`,
+			5,
+		},
+		{
+			`len({a: 1, b: 2, c: 2})`,
+			3,
+		},
 		{
 			`{foo: 0, bar: 1}`,
 			map[string]interface{}{"foo": 0, "bar": 1},
@@ -696,10 +708,6 @@ func TestExpr(t *testing.T) {
 			`(true ? 0+1 : 2+3) + (false ? -1 : -2)`,
 			-1,
 		},
-		{
-			`len(Array)`,
-			5,
-		},
 		{
 			`filter(1..9, {# > 7})`,
 			[]interface{}{8, 9},
diff --git a/vm/runtime.go b/vm/runtime.go
index 9b7f343f9..2dd11ac94 100644
--- a/vm/runtime.go
+++ b/vm/runtime.go
@@ -173,7 +173,7 @@ func in(needle interface{}, array interface{}) bool {
 func length(a interface{}) int {
 	v := reflect.ValueOf(a)
 	switch v.Kind() {
-	case reflect.Array, reflect.Slice, reflect.String:
+	case reflect.Array, reflect.Slice, reflect.Map, reflect.String:
 		return v.Len()
 	default:
 		panic(fmt.Sprintf("invalid argument for len (type %T)", a))