diff --git a/libpyclingo/pyclingo.cc b/libpyclingo/pyclingo.cc
index ec4a33c..88b6669 100644
--- a/libpyclingo/pyclingo.cc
+++ b/libpyclingo/pyclingo.cc
@@ -116,7 +116,7 @@ struct ObjectProtocoll {
     Object call(char const *name, Args &&... args);
     template <class... Args>
     Object operator()(Args &&... args);
-    ssize_t size();
+    Py_ssize_t size();
     bool empty() { return size() == 0; }
     Object getItem(Reference o);
     Object getItem(char const *key);
@@ -232,7 +232,7 @@ Object ObjectProtocoll<T>::operator()(Args &&... args) {
     return PyObject_CallFunctionObjArgs(toPy_(), Reference(args).toPy()..., nullptr);
 }
 template <class T>
-ssize_t ObjectProtocoll<T>::size() {
+Py_ssize_t ObjectProtocoll<T>::size() {
     auto ret = PyObject_Size(toPy_());
     if (PyErr_Occurred()) { throw PyException(); }
     return ret;