diff --git a/datafusion/src/optimizer/simplify_expressions.rs b/datafusion/src/optimizer/simplify_expressions.rs
index 5927810da5512..8e1e7d319e8e7 100644
--- a/datafusion/src/optimizer/simplify_expressions.rs
+++ b/datafusion/src/optimizer/simplify_expressions.rs
@@ -1034,6 +1034,11 @@ mod tests {
         let expr = call_fn("to_timestamp", vec![col("a")]).unwrap();
         test_evaluate(expr.clone(), expr);
 
+        // check that non foldable arguments are folded
+        // to_timestamp(a) --> to_timestamp(a) [no rewrite possible]
+        let expr = call_fn("to_timestamp", vec![col("a")]).unwrap();
+        test_evaluate(expr.clone(), expr);
+
         // volatile / stable functions should not be evaluated
         // rand() + (1 + 2) --> rand() + 3
         let fun = BuiltinScalarFunction::Random;
@@ -1046,7 +1051,6 @@ mod tests {
         // parenthesization matters: can't rewrite
         // (rand() + 1) + 2 --> (rand() + 1) + 2)
         let fun = BuiltinScalarFunction::Random;
-        assert_eq!(fun.volatility(), Volatility::Volatile);
         let rand = Expr::ScalarFunction { args: vec![], fun };
         let expr = (rand + lit(1)) + lit(2);
         test_evaluate(expr.clone(), expr);