diff --git a/cli/doc/printer.rs b/cli/doc/printer.rs
index 7a6b9e731c..f7f41079db 100644
--- a/cli/doc/printer.rs
+++ b/cli/doc/printer.rs
@@ -127,6 +127,9 @@ fn render_params(params: Vec<doc::ParamDef>) -> String {
 }
 
 fn render_ts_type(ts_type: doc::ts_type::TsTypeDef) -> String {
+  if ts_type.kind.is_none() {
+    return "<UNIMPLEMENTED>".to_string();
+  }
   let kind = ts_type.kind.unwrap();
   match kind {
     TsTypeDefKind::Array => {
diff --git a/cli/doc/tests.rs b/cli/doc/tests.rs
index f8e693c9a1..7f2c186396 100644
--- a/cli/doc/tests.rs
+++ b/cli/doc/tests.rs
@@ -161,6 +161,19 @@ export function foo(a: string, b?: number, cb: (...cbArgs: unknown[]) => void, .
   );
 }
 
+#[tokio::test]
+async fn format_type_predicate() {
+  let source_code = r#"
+export function isFish(pet: Fish | Bird): pet is Fish {
+    return (pet as Fish).swim !== undefined;
+}
+"#;
+  let loader =
+    TestLoader::new(vec![("test.ts".to_string(), source_code.to_string())]);
+  let entries = DocParser::new(loader).parse("test.ts").await.unwrap();
+  super::printer::format(entries);
+}
+
 #[tokio::test]
 async fn export_fn2() {
   let source_code = r#"