diff --git a/src/pyexpression/pyexpression.cpp b/src/pyexpression/pyexpression.cpp
index 2a9c38d3..01447f4f 100644
--- a/src/pyexpression/pyexpression.cpp
+++ b/src/pyexpression/pyexpression.cpp
@@ -44,7 +44,7 @@ namespace xo {
 
             py::class_<Expression,
                        rp<Expression>>(m, "Expression")
-                .def("extype", &Expression::extype)
+                .def_property_readonly("extype", &Expression::extype)
                 .def("__repr__", &Expression::display_string);
             ;