diff --git a/src/doc/book/traits.md b/src/doc/book/traits.md
index 107ef2b44d5eb..e685cb129b939 100644
--- a/src/doc/book/traits.md
+++ b/src/doc/book/traits.md
@@ -397,10 +397,10 @@ fn normal<T: ConvertTo<i64>>(x: &T) -> i64 {
 }
 
 // can be called with T == i64
-fn inverse<T>() -> T
+fn inverse<T>(x: i32) -> T
         // this is using ConvertTo as if it were "ConvertTo<i64>"
         where i32: ConvertTo<T> {
-    42.convert()
+    x.convert()
 }
 ```