diff --git a/ucl/builtins.go b/ucl/builtins.go
index e1edbcc..689e09e 100644
--- a/ucl/builtins.go
+++ b/ucl/builtins.go
@@ -283,7 +283,7 @@ func notBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 		return nil, err
 	}
 
-	return BoolObject(!args.args[0].Truthy()), nil
+	return BoolObject(!isTruthy(args.args[0])), nil
 }
 
 var errObjectsNotEqual = errors.New("objects not equal")
diff --git a/ucl/builtins_test.go b/ucl/builtins_test.go
index 4a099f9..8dea035 100644
--- a/ucl/builtins_test.go
+++ b/ucl/builtins_test.go
@@ -1766,6 +1766,35 @@ func TestBuiltins_Cat(t *testing.T) {
 	}
 }
 
+func TestBuiltins_Not(t *testing.T) {
+	tests := []struct {
+		desc string
+		expr string
+		want any
+	}{
+		{desc: "not 1", expr: `not 1`, want: false},
+		{desc: "not 2", expr: `not 0`, want: true},
+		{desc: "not 3", expr: `not ()`, want: true},
+		{desc: "not 4", expr: `not $true`, want: false},
+		{desc: "not 5", expr: `not $false`, want: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := New(WithOut(outW), WithTestBuiltin())
+			inst.SetVar("true", true)
+			inst.SetVar("false", false)
+
+			eqRes, err := inst.EvalString(ctx, tt.expr)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, eqRes)
+		})
+	}
+}
+
 func TestBuiltins_NotNil(t *testing.T) {
 	tests := []struct {
 		desc string