diff --git a/exercises/prime-factors/src/lib.rs b/exercises/prime-factors/src/lib.rs
index e69de29bb..a7628ff7e 100644
--- a/exercises/prime-factors/src/lib.rs
+++ b/exercises/prime-factors/src/lib.rs
@@ -0,0 +1,3 @@
+pub fn factors(n: u32) -> Vec<u32> {
+    unimplemented!("This should calculate the prime factors of {}", n)
+}