diff --git a/datafusion/core/tests/user_defined/user_defined_scalar_functions.rs b/datafusion/core/tests/user_defined/user_defined_scalar_functions.rs
index ad32151ab141..a29ae9d13935 100644
--- a/datafusion/core/tests/user_defined/user_defined_scalar_functions.rs
+++ b/datafusion/core/tests/user_defined/user_defined_scalar_functions.rs
@@ -545,34 +545,39 @@ impl ScalarUDFImpl for CastToI64UDF {
     fn return_type(&self, _args: &[DataType]) -> Result<DataType> {
         Ok(DataType::Int64)
     }
-    // Wrap with Expr::Cast() to Int64
+
+    // Demonstrate simplifying a UDF
     fn simplify(
         &self,
         mut args: Vec<Expr>,
         info: &dyn SimplifyInfo,
     ) -> Result<ExprSimplifyResult> {
+        // DataFusion should have ensured the function is called with just a
+        // single argument
+        assert_eq!(args.len(), 1);
+        let arg = args.pop().unwrap();
+
         // Note that Expr::cast_to requires an ExprSchema but simplify gets a
         // SimplifyInfo so we have to replicate some of the casting logic here.
-        let source_type = info.get_data_type(&args[0])?;
-        if source_type == DataType::Int64 {
-            Ok(ExprSimplifyResult::Original(args))
+
+        let source_type = info.get_data_type(&arg)?;
+        let new_expr = if source_type == DataType::Int64 {
+            // the argument's data type is already the correct type
+            arg
         } else {
-            // DataFusion should have ensured the function is called with just a
-            // single argument
-            assert_eq!(args.len(), 1);
-            let e = args.pop().unwrap();
-            Ok(ExprSimplifyResult::Simplified(Expr::Cast(
-                datafusion_expr::Cast {
-                    expr: Box::new(e),
-                    data_type: DataType::Int64,
-                },
-            )))
-        }
+            // need to use an actual cast to get the correct type
+            Expr::Cast(datafusion_expr::Cast {
+                expr: Box::new(arg),
+                data_type: DataType::Int64,
+            })
+        };
+        // return the newly written argument to DataFusion
+        Ok(ExprSimplifyResult::Simplified(new_expr))
     }
+
     // Casting should be done in `simplify`, so we just return the first argument
-    fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
-        assert_eq!(args.len(), 1);
-        Ok(args.first().unwrap().clone())
+    fn invoke(&self, _args: &[ColumnarValue]) -> Result<ColumnarValue> {
+        unimplemented!("Function should not be evaluated")
     }
 }