diff --git a/sphinx_js/typedoc.py b/sphinx_js/typedoc.py
index b6be16c8..e720c41d 100644
--- a/sphinx_js/typedoc.py
+++ b/sphinx_js/typedoc.py
@@ -410,6 +410,7 @@ def to_ir(self, converter: Converter) -> tuple[ir.Attribute, Sequence["Node"]]:
             # There's no signature to speak of for a getter: only a return type.
             type = self.getSignature.type
         else:
+            assert self.setSignature
             # ES6 says setters have exactly 1 param.
             type = self.setSignature.parameters[0].type