safer and test

src/common.rs

@@ -64,7 +64,11 @@ pub fn invoke<C: FromIterator<Option<I>> + 'static, I>(
     to_array: impl Fn(C) -> DataFusionResult<ArrayRef>,
     to_scalar: impl Fn(Option<I>) -> ScalarValue,
 ) -> DataFusionResult<ColumnarValue> {
-    match &args[0] {
+    let Some(first_arg) = args.first() else {
+        // I think this can't happen, but I assumed the same about args[1] and I was wrong, so better to be safe
+        return exec_err!("expected at least one argument");
+    };
+    match first_arg {
         ColumnarValue::Array(json_array) => {
             let result_collect = match args.get(1) {
                 Some(ColumnarValue::Array(a)) => {

tests/main.rs

@@ -453,3 +453,11 @@ async fn test_json_length_vec() {
     let batches = run_query_large(sql).await.unwrap();
     assert_batches_eq!(expected, &batches);
 }
+
+#[tokio::test]
+async fn test_no_args() {
+    let err = run_query(r#"select json_len()"#).await.unwrap_err();
+    assert!(err
+        .to_string()
+        .contains("No function matches the given name and argument types 'json_length()'."));
+}