diff --git a/query_functions/src/selectors.rs b/query_functions/src/selectors.rs
index f875684a58..5afd930c29 100644
--- a/query_functions/src/selectors.rs
+++ b/query_functions/src/selectors.rs
@@ -464,15 +464,24 @@ fn make_uda(name: &str, factory_builder: FactoryBuilder) -> AggregateUDF {
     //
     // The inputs are (value, time) and the output is a struct with a
     // 'value' and 'time' field of the same time.
+    let captured_name = name.to_string();
     let return_type_func: ReturnTypeFunction = Arc::new(move |arg_types| {
-        assert_eq!(
-            arg_types.len(),
-            2,
-            "selector expected exactly 2 arguments, got {}",
-            arg_types.len()
-        );
+        if arg_types.len() != 2 {
+            return Err(DataFusionError::Plan(format!(
+                "{} requires exactly 2 arguments, got {}",
+                captured_name,
+                arg_types.len()
+            )));
+        }
+
         let input_type = &arg_types[0];
-        assert_eq!(&arg_types[1], &TIME_DATA_TYPE());
+        let time_type = &arg_types[1];
+        if time_type != &TIME_DATA_TYPE() {
+            return Err(DataFusionError::Plan(format!(
+                "{} second argument must be a timestamp, but got {}",
+                captured_name, time_type
+            )));
+        }
         let return_type = output_type.return_type(input_type);
 
         Ok(Arc::new(return_type))
diff --git a/query_tests/src/sql.rs b/query_tests/src/sql.rs
index c11656d11a..b5ab9ab0b8 100644
--- a/query_tests/src/sql.rs
+++ b/query_tests/src/sql.rs
@@ -362,4 +362,26 @@ async fn sql_create_schema() {
     .await;
 }
 
+#[tokio::test]
+async fn bad_selector_num_args() {
+    let expected_error = "selector_last requires exactly 2 arguments, got 1";
+    run_sql_error_test_case(
+        scenarios::delete::NoDeleteOneChunk {},
+        "select selector_last(time)['bar'] from cpu;",
+        expected_error,
+    )
+    .await;
+}
+
+#[tokio::test]
+async fn bad_selector_arg_types() {
+    let expected_error = "selector_last second argument must be a timestamp, but got Float64";
+    run_sql_error_test_case(
+        scenarios::delete::NoDeleteOneChunk {},
+        "select selector_last(time, bar)['value'] from cpu;",
+        expected_error,
+    )
+    .await;
+}
+
 // --------------------------------------------------------