diff --git a/builtin/builtin.go b/builtin/builtin.go
index cd943bd0..14fc8a4b 100644
--- a/builtin/builtin.go
+++ b/builtin/builtin.go
@@ -12,16 +12,6 @@ import (
 	"github.com/expr-lang/expr/vm/runtime"
 )
 
-type Function struct {
-	Name         string
-	Func         func(args ...any) (any, error)
-	Fast         func(arg any) any
-	ValidateArgs func(args ...any) (any, error)
-	Types        []reflect.Type
-	Validate     func(args []reflect.Type) (reflect.Type, error)
-	Predicate    bool
-}
-
 var (
 	Index map[string]int
 	Names []string
diff --git a/builtin/function.go b/builtin/function.go
new file mode 100644
index 00000000..e6d88234
--- /dev/null
+++ b/builtin/function.go
@@ -0,0 +1,22 @@
+package builtin
+
+import (
+	"reflect"
+)
+
+type Function struct {
+	Name         string
+	Func         func(args ...any) (any, error)
+	Fast         func(arg any) any
+	ValidateArgs func(args ...any) (any, error)
+	Types        []reflect.Type
+	Validate     func(args []reflect.Type) (reflect.Type, error)
+	Predicate    bool
+}
+
+func (f *Function) Type() reflect.Type {
+	if len(f.Types) > 0 {
+		return f.Types[0]
+	}
+	return reflect.TypeOf(f.Func)
+}
diff --git a/checker/checker.go b/checker/checker.go
index 5dd722fe..0b5a0227 100644
--- a/checker/checker.go
+++ b/checker/checker.go
@@ -169,10 +169,10 @@ func (v *checker) ident(node ast.Node, name string, strict, builtins bool) (refl
 	}
 	if builtins {
 		if fn, ok := v.config.Functions[name]; ok {
-			return functionType, info{fn: fn}
+			return fn.Type(), info{fn: fn}
 		}
 		if fn, ok := v.config.Builtins[name]; ok {
-			return functionType, info{fn: fn}
+			return fn.Type(), info{fn: fn}
 		}
 	}
 	if v.config.Strict && strict {
@@ -833,7 +833,7 @@ func (v *checker) checkFunction(f *builtin.Function, node ast.Node, arguments []
 		}
 		return t, info{}
 	} else if len(f.Types) == 0 {
-		t, err := v.checkArguments(f.Name, functionType, false, arguments, node)
+		t, err := v.checkArguments(f.Name, f.Type(), false, arguments, node)
 		if err != nil {
 			if v.err == nil {
 				v.err = err
diff --git a/checker/checker_test.go b/checker/checker_test.go
index d03e3a8e..35daeda4 100644
--- a/checker/checker_test.go
+++ b/checker/checker_test.go
@@ -994,7 +994,7 @@ func TestCheck_builtin_without_call(t *testing.T) {
 		err   string
 	}{
 		{`len + 1`, "invalid operation: + (mismatched types func(...interface {}) (interface {}, error) and int) (1:5)\n | len + 1\n | ....^"},
-		{`string.A`, "type func(...interface {}) (interface {}, error)[string] is undefined (1:8)\n | string.A\n | .......^"},
+		{`string.A`, "type func(interface {}) string[string] is undefined (1:8)\n | string.A\n | .......^"},
 	}
 
 	for _, test := range tests {
diff --git a/checker/types.go b/checker/types.go
index 662139c3..8c080504 100644
--- a/checker/types.go
+++ b/checker/types.go
@@ -18,7 +18,6 @@ var (
 	anyType      = reflect.TypeOf(new(any)).Elem()
 	timeType     = reflect.TypeOf(time.Time{})
 	durationType = reflect.TypeOf(time.Duration(0))
-	functionType = reflect.TypeOf(new(func(...any) (any, error))).Elem()
 )
 
 func combined(a, b reflect.Type) reflect.Type {
diff --git a/patcher/with_context_test.go b/patcher/with_context_test.go
index 0d0917af..4c2bd048 100644
--- a/patcher/with_context_test.go
+++ b/patcher/with_context_test.go
@@ -30,3 +30,34 @@ func TestWithContext(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 42, output)
 }
+
+func TestWithContext_with_env_Function(t *testing.T) {
+	env := map[string]any{
+		"ctx": context.TODO(),
+	}
+
+	fn := expr.Function("fn",
+		func(params ...any) (any, error) {
+			ctx := params[0].(context.Context)
+			a := params[1].(int)
+
+			return ctx.Value("value").(int) + a, nil
+		},
+		new(func(context.Context, int) int),
+	)
+
+	program, err := expr.Compile(
+		`fn(40)`,
+		expr.Env(env),
+		expr.WithContext("ctx"),
+		fn,
+	)
+	require.NoError(t, err)
+
+	ctx := context.WithValue(context.Background(), "value", 2)
+	env["ctx"] = ctx
+
+	output, err := expr.Run(program, env)
+	require.NoError(t, err)
+	require.Equal(t, 42, output)
+}