diff --git a/src/SIM/SIMbase.C b/src/SIM/SIMbase.C
index 6e7449c64..f9f0a0ed5 100644
--- a/src/SIM/SIMbase.C
+++ b/src/SIM/SIMbase.C
@@ -2136,6 +2136,7 @@ bool SIMbase::project (RealArray& values, const FunctionBase* f,
   for (size_t j = 0; j < myModel.size() && ok; j++)
   {
     if (myModel[j]->empty()) continue; // skip empty patches
+    f->initPatch(j);
 
     Vector loc_values;
     switch (pMethod) {
diff --git a/src/Utility/FieldFunctions.C b/src/Utility/FieldFunctions.C
index bd61fc17c..eaa337eca 100644
--- a/src/Utility/FieldFunctions.C
+++ b/src/Utility/FieldFunctions.C
@@ -36,7 +36,7 @@ FieldFuncBase::~FieldFuncBase ()
 }
 
 
-bool FieldFuncBase::setPatch (size_t pIdx)
+bool FieldFuncBase::setPatch (size_t pIdx) const
 {
   if (pIdx >= npch)
     return false;
diff --git a/src/Utility/FieldFunctions.h b/src/Utility/FieldFunctions.h
index ccd519d77..37c2070da 100644
--- a/src/Utility/FieldFunctions.h
+++ b/src/Utility/FieldFunctions.h
@@ -42,12 +42,12 @@ class FieldFuncBase
   virtual ~FieldFuncBase();
 
   //! \brief Sets the active patch.
-  bool setPatch(size_t pIdx);
+  bool setPatch(size_t pIdx) const;
 
 protected:
   std::vector<ASMbase*> patch; //!< The patches on which the field is defined
 
-  size_t pidx; //!< Current patch index
+  mutable size_t pidx; //!< Current patch index
   size_t npch; //!< Number of patches in the field
 };
 
@@ -170,7 +170,7 @@ class FieldFunction : public RealFunc, public FieldFuncScalarBase
   virtual ~FieldFunction() {}
 
   //! \brief Sets the active patch.
-  virtual bool initPatch(size_t pIdx) { return this->setPatch(pIdx); }
+  virtual bool initPatch(size_t pIdx) const { return this->setPatch(pIdx); }
 
 protected:
   //! \brief Evaluates the scalar field function.
diff --git a/src/Utility/Function.h b/src/Utility/Function.h
index 5e9e49d55..81bfe9e58 100644
--- a/src/Utility/Function.h
+++ b/src/Utility/Function.h
@@ -160,7 +160,7 @@ class FunctionBase
   size_t dim() const { return ncmp; }
 
   //! \brief Sets the active patch.
-  virtual bool initPatch(size_t) { return true; }
+  virtual bool initPatch(size_t) const { return true; }
 
   //! \brief Checks if a specified point is within the function domain.
   virtual bool inDomain(const Vec3&) const { return true; }