diff --git a/modules/conop/src/conop.cc b/modules/conop/src/conop.cc
index 138d62c7c988e11c6393cca79d6b222fd9c9c239..f72956f1a2f732a5734ddd1a8d20bec46e5cc91f 100644
--- a/modules/conop/src/conop.cc
+++ b/modules/conop/src/conop.cc
@@ -161,7 +161,8 @@ void Conopology::SetDefaultBuilder(const String& default_name) {
   if (builder)
     builder_map_["DEFAULT"]=builder;
   else
-    assert(0 && "what is this?");
+    throw std::runtime_error("trying to set unknown builder '"+
+                             default_name+"' as the default");
 }
 
 BuilderP Conopology::GetBuilder(const String& name)