diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index 625bea8..904baf1 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -111,7 +111,11 @@ func (ca CallArgs) bindArg(v interface{}, arg object) error {
 		}
 		return nil
 	case *string:
-		*t = arg.String()
+		if arg != nil {
+			*t = arg.String()
+		} else {
+			*t = ""
+		}
 	case *int:
 		if iArg, ok := arg.(intObject); ok {
 			*t = int(iArg)
diff --git a/ucl/userbuiltin_test.go b/ucl/userbuiltin_test.go
index a4bd231..557a833 100644
--- a/ucl/userbuiltin_test.go
+++ b/ucl/userbuiltin_test.go
@@ -81,6 +81,7 @@ func TestInst_SetBuiltin(t *testing.T) {
 			{"plain eval", `add2 -sep ", " -right "world" -left "Hello"`, "Hello, world"},
 			{"swap 1", `add2 -right "right" -left "left" -sep ":"`, "left:right"},
 			{"swap 2", `add2 -left "left" -sep ":" -right "right" -upcase`, "LEFT:RIGHT"},
+			{"nil 1", `add2 -right "right" -left () -sep ":"`, ":right"},
 		}
 
 		for _, tt := range tests {
@@ -186,32 +187,66 @@ func TestInst_SetBuiltin(t *testing.T) {
 }
 
 func TestCallArgs_Bind(t *testing.T) {
-	ctx := context.Background()
+	t.Run("happy path", func(t *testing.T) {
+		ctx := context.Background()
 
-	inst := ucl.New()
-	inst.SetBuiltin("sa", func(ctx context.Context, args ucl.CallArgs) (any, error) {
-		return doStringA{this: "a val"}, nil
-	})
-	inst.SetBuiltin("sb", func(ctx context.Context, args ucl.CallArgs) (any, error) {
-		return doStringB{left: "foo", right: "bar"}, nil
-	})
-	inst.SetBuiltin("dostr", func(ctx context.Context, args ucl.CallArgs) (any, error) {
-		var ds doStringable
+		inst := ucl.New()
+		inst.SetBuiltin("sa", func(ctx context.Context, args ucl.CallArgs) (any, error) {
+			return doStringA{this: "a val"}, nil
+		})
+		inst.SetBuiltin("sb", func(ctx context.Context, args ucl.CallArgs) (any, error) {
+			return doStringB{left: "foo", right: "bar"}, nil
+		})
+		inst.SetBuiltin("dostr", func(ctx context.Context, args ucl.CallArgs) (any, error) {
+			var ds doStringable
 
-		if err := args.Bind(&ds); err != nil {
-			return nil, err
+			if err := args.Bind(&ds); err != nil {
+				return nil, err
+			}
+
+			return ds.DoString(), nil
+		})
+
+		va, err := inst.Eval(ctx, `dostr (sa)`)
+		assert.NoError(t, err)
+		assert.Equal(t, "do string A: a val", va)
+
+		vb, err := inst.Eval(ctx, `dostr (sb)`)
+		assert.NoError(t, err)
+		assert.Equal(t, "do string B: foo bar", vb)
+	})
+
+	t.Run("bind to string", func(t *testing.T) {
+		tests := []struct {
+			descr string
+			eval  string
+			want  string
+		}{
+			{descr: "string value", eval: `sa "hello"`, want: "[hello]"},
+			{descr: "int value", eval: `sa 13245`, want: "[13245]"},
+			{descr: "nil value", eval: `sa ()`, want: "[]"},
 		}
 
-		return ds.DoString(), nil
+		for _, tt := range tests {
+			t.Run(tt.descr, func(t *testing.T) {
+				ctx := context.Background()
+
+				inst := ucl.New()
+				inst.SetBuiltin("sa", func(ctx context.Context, args ucl.CallArgs) (any, error) {
+					var v string
+					if err := args.Bind(&v); err != nil {
+						return nil, err
+					}
+
+					return fmt.Sprintf("[%v]", v), nil
+				})
+
+				res, err := inst.Eval(ctx, tt.eval)
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, res)
+			})
+		}
 	})
-
-	va, err := inst.Eval(ctx, `dostr (sa)`)
-	assert.NoError(t, err)
-	assert.Equal(t, "do string A: a val", va)
-
-	vb, err := inst.Eval(ctx, `dostr (sb)`)
-	assert.NoError(t, err)
-	assert.Equal(t, "do string B: foo bar", vb)
 }
 
 func TestCallArgs_CanBind(t *testing.T) {