diff --git a/src/goto-programs/class_hierarchy.cpp b/src/goto-programs/class_hierarchy.cpp
index cb9106c818d..db7cb2c5d90 100644
--- a/src/goto-programs/class_hierarchy.cpp
+++ b/src/goto-programs/class_hierarchy.cpp
@@ -50,8 +50,11 @@ void class_hierarchy_grapht::populate(const symbol_tablet &symbol_table)
         const irep_idt &parent = to_symbol_type(base.type()).get_identifier();
         if(!parent.empty())
         {
-          add_edge(
-            nodes_by_name.at(parent), nodes_by_name.at(symbol_pair.first));
+          const auto parent_node_it = nodes_by_name.find(parent);
+          DATA_INVARIANT(
+            parent_node_it != nodes_by_name.end(),
+            "parent class not in symbol table");
+          add_edge(parent_node_it->second, nodes_by_name.at(symbol_pair.first));
         }
       }
     }