diff --git a/exercises/option/option1.rs b/exercises/option/option1.rs
index 51c39f5..2a6c2f7 100644
--- a/exercises/option/option1.rs
+++ b/exercises/option/option1.rs
@@ -18,6 +18,6 @@ fn main() {
             ((iter * 5) + 2) / (4 * 16)
         };
 
-        numbers[iter] = number_to_add;
+        numbers[iter as usize] = number_to_add;
     }
 }