diff --git a/script/conversion.go b/script/conversion.go
index 6dbda2a..c315434 100644
--- a/script/conversion.go
+++ b/script/conversion.go
@@ -37,7 +37,7 @@ func objectToString(o objects.Object) string {
 		return val.Value
 	}
 
-	return ""
+	return o.String()
 }
 
 func objectToInterface(o objects.Object) interface{} {
@@ -56,9 +56,11 @@ func objectToInterface(o objects.Object) interface{} {
 		return val.Value
 	case *objects.String:
 		return val.Value
+	case *objects.Undefined:
+		return nil
 	}
 
-	return nil
+	return o
 }
 
 func interfaceToObject(v interface{}) (objects.Object, error) {
diff --git a/script/script_custom_test.go b/script/script_custom_test.go
new file mode 100644
index 0000000..d8b0a9f
--- /dev/null
+++ b/script/script_custom_test.go
@@ -0,0 +1,101 @@
+package script_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/d5/tengo/assert"
+	"github.com/d5/tengo/compiler/token"
+	"github.com/d5/tengo/objects"
+	"github.com/d5/tengo/script"
+)
+
+type Counter struct {
+	value int64
+}
+
+func (o *Counter) TypeName() string {
+	return "counter"
+}
+
+func (o *Counter) String() string {
+	return fmt.Sprintf("Counter(%d)", o.value)
+}
+
+func (o *Counter) BinaryOp(op token.Token, rhs objects.Object) (objects.Object, error) {
+	switch rhs := rhs.(type) {
+	case *Counter:
+		switch op {
+		case token.Add:
+			return &Counter{value: o.value + rhs.value}, nil
+		case token.Sub:
+			return &Counter{value: o.value - rhs.value}, nil
+		}
+	case *objects.Int:
+		switch op {
+		case token.Add:
+			return &Counter{value: o.value + rhs.Value}, nil
+		case token.Sub:
+			return &Counter{value: o.value - rhs.Value}, nil
+		}
+	}
+
+	return nil, errors.New("invalid operator")
+}
+
+func (o *Counter) IsFalsy() bool {
+	return o.value == 0
+}
+
+func (o *Counter) Equals(t objects.Object) bool {
+	if tc, ok := t.(*Counter); ok {
+		return o.value == tc.value
+	}
+
+	return false
+}
+
+func (o *Counter) Copy() objects.Object {
+	return &Counter{value: o.value}
+}
+
+func (o *Counter) Call(args ...objects.Object) (objects.Object, error) {
+	return &objects.Int{Value: o.value}, nil
+}
+
+func TestScript_CustomObjects(t *testing.T) {
+	c := compile(t, `a := c1(); s := string(c1); c2 := c1; c2++`, M{
+		"c1": &Counter{value: 5},
+	})
+	compiledRun(t, c)
+	compiledGet(t, c, "a", int64(5))
+	compiledGet(t, c, "s", "Counter(5)")
+	compiledGetCounter(t, c, "c2", &Counter{value: 6})
+
+	c = compile(t, `
+arr := [1, 2, 3, 4]
+for x in arr {
+	c1 += x
+}
+out := c1()
+`, M{
+		"c1": &Counter{value: 5},
+	})
+	compiledRun(t, c)
+	compiledGet(t, c, "out", int64(15))
+}
+
+func compiledGetCounter(t *testing.T, c *script.Compiled, name string, expected *Counter) bool {
+	v := c.Get(name)
+	if !assert.NotNil(t, v) {
+		return false
+	}
+
+	actual := v.Value().(*Counter)
+	if !assert.NotNil(t, actual) {
+		return false
+	}
+
+	return assert.Equal(t, expected.value, actual.value)
+}