diff --git a/library/core/src/iter/sources/repeat.rs b/library/core/src/iter/sources/repeat.rs
index 4bcd5b16aea6..ac218e9b617a 100644
--- a/library/core/src/iter/sources/repeat.rs
+++ b/library/core/src/iter/sources/repeat.rs
@@ -97,8 +97,9 @@ fn nth(&mut self, n: usize) -> Option<A> {
         Some(self.element.clone())
     }
 
+    #[track_caller]
     fn last(self) -> Option<A> {
-        Some(self.element)
+        panic!("iterator is infinite");
     }
 
     #[track_caller]
diff --git a/library/coretests/tests/iter/sources.rs b/library/coretests/tests/iter/sources.rs
index 5a391cb67751..420f3088e6ee 100644
--- a/library/coretests/tests/iter/sources.rs
+++ b/library/coretests/tests/iter/sources.rs
@@ -37,6 +37,7 @@ fn test_repeat_count() {
 }
 
 #[test]
+#[should_panic = "iterator is infinite"]
 fn test_repeat_last() {
     assert_eq!(repeat(42).last(), Some(42));
 }