diff --git a/exercises/standard_library_types/iterators4.rs b/exercises/standard_library_types/iterators4.rs
index 8886283..0b3b5d8 100644
--- a/exercises/standard_library_types/iterators4.rs
+++ b/exercises/standard_library_types/iterators4.rs
@@ -18,6 +18,11 @@ pub fn factorial(num: u64) -> u64 {
 mod tests {
     use super::*;
 
+    #[test]
+    fn factorial_of_0() {
+        assert_eq!(1, factorial(0));
+    }
+
     #[test]
     fn factorial_of_1() {
         assert_eq!(1, factorial(1));