diff --git a/evaluator_test.go b/evaluator_test.go
index fdd191e..719c84e 100644
--- a/evaluator_test.go
+++ b/evaluator_test.go
@@ -295,13 +295,13 @@ func TestProperties(t *testing.T) {
 		{
 			expr: `(or
 	(and
-	(between age 18 80)
+	(between AGE 18 80)
 	(eq gender "male")
 	(between app_version (t_version "2.7.1") (t_version "2.9.1"))
 	)
 	(overlap region (2890 3780))
 	 )`,
- 			res: []string{"age", "gender", "app_version", "region"},
+			res: []string{"AGE", "gender", "app_version", "region"},
 		},
 		{
 			expr: `(and
@@ -311,6 +311,14 @@ func TestProperties(t *testing.T) {
 	)`,
 			res: []string{"os", "app_version", "os", "affiliate", "os", "language"},
 		},
+		{
+			expr: `(eq 1 1)`,
+			res:  nil,
+		},
+		{
+			expr: `(and () ())`,
+			res:  nil,
+		},
 	}
 	for _, input := range inputs {
 		e, err := New(input.expr)
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..7072c04
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/nullne/evaluator
+
+go 1.14