diff --git a/Grid/qcd/action/fermion/FermionOperator.h b/Grid/qcd/action/fermion/FermionOperator.h
index 0c159300..66644d7f 100644
--- a/Grid/qcd/action/fermion/FermionOperator.h
+++ b/Grid/qcd/action/fermion/FermionOperator.h
@@ -49,7 +49,7 @@ public:
 
   virtual FermionField &tmp(void) = 0;
 
-  virtual void DirichletBlock(Coordinate & _Block) { assert(0); };
+  virtual void DirichletBlock(const Coordinate & _Block) { assert(0); };
   
   GridBase * Grid(void)   { return FermionGrid(); };   // this is all the linalg routines need to know
   GridBase * RedBlackGrid(void) { return FermionRedBlackGrid(); };