diff --git a/ucl/eval.go b/ucl/eval.go
index 237864b..98e352e 100644
--- a/ucl/eval.go
+++ b/ucl/eval.go
@@ -186,7 +186,16 @@ func (e evaluator) evalArg(ctx context.Context, ec *evalCtx, n astCmdArg) (Objec
 		if v, ok := ec.getVar(*n.Var); ok {
 			return v, nil
 		}
-		return nil, nil
+
+		if e.inst.missingVarHandler != nil {
+			dv, err := e.inst.missingVarHandler(ctx, *n.Var)
+			if err != nil {
+				return nil, err
+			}
+			return fromGoValue(dv)
+		}
+
+		return nil, errors.New("undefined variable: " + *n.Var)
 	case n.MaybeSub != nil:
 		sub := n.MaybeSub.Sub
 		if sub == nil {
diff --git a/ucl/inst.go b/ucl/inst.go
index 550975d..5414753 100644
--- a/ucl/inst.go
+++ b/ucl/inst.go
@@ -11,6 +11,7 @@ import (
 type Inst struct {
 	out                   io.Writer
 	missingBuiltinHandler MissingBuiltinHandler
+	missingVarHandler     MissingVarHandler
 	echoPrinter           EchoPrinter
 
 	rootEC *evalCtx
@@ -30,6 +31,12 @@ func WithMissingBuiltinHandler(handler MissingBuiltinHandler) InstOption {
 	}
 }
 
+func WithMissingVarHandler(handler MissingVarHandler) InstOption {
+	return func(i *Inst) {
+		i.missingVarHandler = handler
+	}
+}
+
 func WithModule(module Module) InstOption {
 	return func(i *Inst) {
 		for name, builtin := range module.Builtins {
diff --git a/ucl/inst_test.go b/ucl/inst_test.go
index 1da27e5..1faa97c 100644
--- a/ucl/inst_test.go
+++ b/ucl/inst_test.go
@@ -119,6 +119,64 @@ func TestInst_Eval(t *testing.T) {
 	}
 }
 
+func TestInst_MissingVarHandler(t *testing.T) {
+	tests := []struct {
+		desc    string
+		expr    string
+		handler ucl.MissingVarHandler
+		want    any
+		wantErr string
+	}{
+		{
+			desc:    "default - error",
+			expr:    `$bla`,
+			wantErr: "undefined variable: bla",
+		},
+		{
+			desc: "handler - set to nil",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			},
+			expr: `$bla`,
+			want: nil,
+		},
+		{
+			desc: "handler - set to a string 1",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return "I am " + name, nil
+			},
+			expr: `$bla`,
+			want: "I am bla",
+		},
+		{
+			desc: "handler - set to a string 2",
+			handler: func(ctx context.Context, name string) (any, error) {
+				return "I am " + name, nil
+			},
+			expr: `$something`,
+			want: "I am something",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := ucl.New(ucl.WithOut(outW), ucl.WithTestBuiltin(), ucl.WithMissingVarHandler(tt.handler))
+			res, err := inst.Eval(ctx, tt.expr)
+
+			if tt.wantErr != "" {
+				assert.Error(t, err)
+				assert.Equal(t, err.Error(), tt.wantErr)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, res)
+			}
+		})
+	}
+}
+
 var parseComments1 = `
 	proc lookup { |file|
 		foreach { |toks|
diff --git a/ucl/testbuiltins_test.go b/ucl/testbuiltins_test.go
index 26e15e7..917a4ac 100644
--- a/ucl/testbuiltins_test.go
+++ b/ucl/testbuiltins_test.go
@@ -197,7 +197,9 @@ func TestBuiltins_If(t *testing.T) {
 			ctx := context.Background()
 			outW := bytes.NewBuffer(nil)
 
-			inst := New(WithOut(outW), WithTestBuiltin())
+			inst := New(WithOut(outW), WithTestBuiltin(), WithMissingVarHandler(func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			}))
 			err := evalAndDisplay(ctx, inst, tt.expr)
 
 			assert.NoError(t, err)
@@ -387,7 +389,9 @@ func TestBuiltins_Try(t *testing.T) {
 			ctx := context.Background()
 			outW := bytes.NewBuffer(nil)
 
-			inst := New(WithOut(outW), WithTestBuiltin())
+			inst := New(WithOut(outW), WithTestBuiltin(), WithMissingVarHandler(func(ctx context.Context, name string) (any, error) {
+				return nil, nil
+			}))
 			err := evalAndDisplay(ctx, inst, tt.expr)
 
 			if tt.wantErr != "" {
diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index ce26e40..5e2ee07 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -11,6 +11,7 @@ import (
 type BuiltinHandler func(ctx context.Context, args CallArgs) (any, error)
 
 type MissingBuiltinHandler func(ctx context.Context, name string, args CallArgs) (any, error)
+type MissingVarHandler func(ctx context.Context, name string) (any, error)
 
 type CallArgs struct {
 	args invocationArgs