diff --git a/src/items.rs b/src/items.rs
index 562986da6a9..25ed376f277 100644
--- a/src/items.rs
+++ b/src/items.rs
@@ -2740,6 +2740,8 @@ impl Rewrite for ast::ForeignItem {
                     format!("{}{}{};", prefix, sep, ty_str)
                 })
             }
+            // FIXME(#2097) support extern types.
+            ast::ForeignItemKind::Ty => unimplemented!(),
         }?;
 
         let missing_span = if self.attrs.is_empty() {