diff --git a/Solver/dofManager.h b/Solver/dofManager.h
index 344930b6e2f97229aa47825b172d71236eeddbd4..4783b3eee3b259df5a19110e13e1c56e2452ba7d 100644
--- a/Solver/dofManager.h
+++ b/Solver/dofManager.h
@@ -548,7 +548,7 @@ class dofManager{
     }
   }
 
-  int getDofNumber(Dof& key){
+  int getDofNumber(const Dof& key){
 		std::map<Dof,int>::iterator it = unknown.find(key);
 		if (it == unknown.end()) {
 			return -1;