diff --git a/src/selector/mod.rs b/src/selector/mod.rs
index 97b341c..4bce1f1 100644
--- a/src/selector/mod.rs
+++ b/src/selector/mod.rs
@@ -316,7 +316,7 @@ impl Selector {
                     }
                     continue;
                 }
-                _ => todo!(),
+                _ => return Err("expected selector.".into()),
             });
             iter.next();
         }