diff --git a/exercises/18_iterators/iterators4.rs b/exercises/18_iterators/iterators4.rs
index 08ba3650..8381dbb1 100644
--- a/exercises/18_iterators/iterators4.rs
+++ b/exercises/18_iterators/iterators4.rs
@@ -1,4 +1,4 @@
-fn factorial(num: u8) -> u64 {
+fn factorial(num: u64) -> u64 {
     // TODO: Complete this function to return the factorial of `num`.
     // Do not use:
     // - early returns (using the `return` keyword explicitly)