diff --git a/src/doc/book/error-handling.md b/src/doc/book/error-handling.md
index 11086af10bd3d..12cb71973ab25 100644
--- a/src/doc/book/error-handling.md
+++ b/src/doc/book/error-handling.md
@@ -2019,6 +2019,16 @@ impl Error for CliError {
             CliError::NotFound => "not found",
         }
     }
+
+    fn cause(&self) -> Option<&error::Error> {
+        match *self {            
+            CliError::Io(ref err) => Some(err),
+            CliError::Parse(ref err) => Some(err),
+            // Our custom error doesn't have an underlying cause, but we could
+            // modify it so that it does.
+            CliError::NotFound() => None,
+        }
+    }
 }
 ```