diff --git a/modules/conop/src/processor.cc b/modules/conop/src/processor.cc
index a1d6a6cfb2c844ee03159901abbf00953d551792..d8f5c269837dd4f38a892b5fc8996ee4e9986656 100644
--- a/modules/conop/src/processor.cc
+++ b/modules/conop/src/processor.cc
@@ -427,6 +427,8 @@ String StringFromConopAction(ConopAction action) {
     case CONOP_WARN:
       return "warn";
   }
+  assert(0 && "unhandled conop action");
+  return "unhandled conop action";
 }
 
 bool Processor::AreResiduesConsecutive(mol::ResidueHandle r1,