diff --git a/checker/checker.go b/checker/checker.go
index 3e320621d..bd9a6d4a7 100644
--- a/checker/checker.go
+++ b/checker/checker.go
@@ -28,6 +28,7 @@ func Check(tree *parser.Tree, config *conf.Config) (t reflect.Type, err error) {
 		v.types = config.Types
 		v.operators = config.Operators
 		v.expect = config.Expect
+		v.undefVars = config.AllowUndefinedVariables
 	}
 
 	t = v.visit(tree.Node)
@@ -55,6 +56,7 @@ type visitor struct {
 	operators   conf.OperatorsTable
 	expect      reflect.Kind
 	collections []reflect.Type
+	undefVars   bool
 }
 
 func (v *visitor) visit(node ast.Node) reflect.Type {
@@ -127,6 +129,9 @@ func (v *visitor) IdentifierNode(node *ast.IdentifierNode) reflect.Type {
 	if t, ok := v.types[node.Value]; ok {
 		return t.Type
 	}
+	if v.undefVars {
+		return interfaceType
+	}
 	panic(v.error(node, "unknown name %v", node.Value))
 }
 
diff --git a/expr.go b/expr.go
index d65a06f50..191e173d0 100644
--- a/expr.go
+++ b/expr.go
@@ -49,10 +49,22 @@ func Env(i interface{}) Option {
 		if _, ok := i.(map[string]interface{}); ok {
 			c.MapEnv = true
 		}
+		c.CheckTypes = true
 		c.Types = conf.CreateTypesTable(i)
 	}
 }
 
+// AllowUndefinedVariables allows to use undefined variables inside expressions.
+// This can be used with expr.Env option to partially define a few variables.
+// Note what this option is only works in map environment are used, otherwise
+// runtime.fetch will panic as there is no way to get missing field zero value.
+func AllowUndefinedVariables() Option {
+	return func(c *conf.Config) {
+		c.CheckTypes = true
+		c.AllowUndefinedVariables = true
+	}
+}
+
 // Operator allows to override binary operator with function.
 func Operator(operator string, fn ...string) Option {
 	return func(c *conf.Config) {
@@ -108,7 +120,7 @@ func Compile(input string, ops ...Option) (*vm.Program, error) {
 		return nil, err
 	}
 
-	if config.Types != nil {
+	if config.CheckTypes {
 		_, err = checker.Check(tree, config)
 		if err != nil {
 			return nil, err
diff --git a/expr_test.go b/expr_test.go
index 378c4ef0c..fbd42dd75 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -1,14 +1,12 @@
 package expr_test
 
 import (
-	"encoding/json"
 	"fmt"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/antonmedv/expr"
-	"github.com/antonmedv/expr/vm"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -209,6 +207,33 @@ func ExampleEnv_with_undefined_variables() {
 	// Output: 5
 }
 
+func ExampleEnv_allow_undefined_variables() {
+	env := map[string]string{
+		"greet": "",
+	}
+
+	program, err := expr.Compile(`greet + name`, expr.Env(env), expr.AllowUndefinedVariables())
+	if err != nil {
+		fmt.Printf("%v", err)
+		return
+	}
+
+	params := map[string]string{
+		"greet": "hello, ",
+		"name":  "world",
+	}
+
+	output, err := expr.Run(program, params)
+	if err != nil {
+		fmt.Printf("%v", err)
+		return
+	}
+
+	fmt.Printf("%v", output)
+
+	// Output: hello, world
+}
+
 func ExampleAsBool() {
 	env := map[string]int{
 		"foo": 0,
@@ -359,42 +384,6 @@ func ExampleOperator_time() {
 	// Output: true
 }
 
-func ExampleEval_marshal() {
-	env := map[string]int{
-		"foo": 1,
-		"bar": 2,
-	}
-
-	program, err := expr.Compile("(foo + bar) in [1, 2, 3]", expr.Env(env))
-	if err != nil {
-		fmt.Printf("%v", err)
-		return
-	}
-
-	b, err := json.Marshal(program)
-	if err != nil {
-		fmt.Printf("%v", err)
-		return
-	}
-
-	unmarshaledProgram := &vm.Program{}
-	err = json.Unmarshal(b, unmarshaledProgram)
-	if err != nil {
-		fmt.Printf("%v", err)
-		return
-	}
-
-	output, err := expr.Run(unmarshaledProgram, env)
-	if err != nil {
-		fmt.Printf("%v", err)
-		return
-	}
-
-	fmt.Printf("%v", output)
-
-	// Output: true
-}
-
 func TestOperator_struct(t *testing.T) {
 	env := &mockEnv{
 		BirthDay: time.Date(2017, time.October, 23, 18, 30, 0, 0, time.UTC),
diff --git a/internal/conf/config.go b/internal/conf/config.go
index d22972393..a0086e150 100644
--- a/internal/conf/config.go
+++ b/internal/conf/config.go
@@ -6,11 +6,13 @@ import (
 )
 
 type Config struct {
-	MapEnv    bool
-	Types     TypesTable
-	Operators OperatorsTable
-	Expect    reflect.Kind
-	Optimize  bool
+	MapEnv                  bool
+	Types                   TypesTable
+	CheckTypes              bool
+	Operators               OperatorsTable
+	Expect                  reflect.Kind
+	Optimize                bool
+	AllowUndefinedVariables bool
 }
 
 func New(i interface{}) *Config {