diff --git a/src/ExprNode.cc b/src/ExprNode.cc
index dc8e9392692de76c3867ac1f25638eb5e95a57e5..afd4cce4bb22a39dde52c27460666332fded730a 100644
--- a/src/ExprNode.cc
+++ b/src/ExprNode.cc
@@ -9588,7 +9588,9 @@ PacExpectationNode::writeJsonOutput(ostream &output,
                                     const deriv_node_temp_terms_t &tef_terms,
                                     const bool isdynamic) const
 {
-  output << "pac_expectation(" << model_name << ')';
+  output << "pac_expectation("
+         << "model_name = " << model_name
+         << ")";
 }
 
 bool