diff --git a/schemars_derive/src/ast/mod.rs b/schemars_derive/src/ast/mod.rs
index 9688878..e0ac3fc 100644
--- a/schemars_derive/src/ast/mod.rs
+++ b/schemars_derive/src/ast/mod.rs
@@ -73,7 +73,7 @@ impl<'a> Field<'a> {
         match &self.attrs.with {
             None => self.ty,
             Some(WithAttr::Type(ty)) => ty,
-            Some(WithAttr::_Function(_)) => todo!(),
+            Some(WithAttr::_Function(_)) => unimplemented!(), // TODO
         }
     }
 }