diff --git a/ProcessLib/Output/SecondaryVariable.cpp b/ProcessLib/Output/SecondaryVariable.cpp
index 604b152d1d83dde9ff262114cbd342b2cff4ebcb..b167f810a10c48f04c7d7a0b1c5e5d815e3ea14b 100644
--- a/ProcessLib/Output/SecondaryVariable.cpp
+++ b/ProcessLib/Output/SecondaryVariable.cpp
@@ -39,19 +39,19 @@ void SecondaryVariableCollection::addSecondaryVariable(
 }
 
 std::map<std::string, std::string>::const_iterator
-SecondaryVariableCollection::begin()
+SecondaryVariableCollection::begin() const
 {
     return _map_external_to_internal.cbegin();
 }
 
 std::map<std::string, std::string>::const_iterator
-SecondaryVariableCollection::end()
+SecondaryVariableCollection::end() const
 {
     return _map_external_to_internal.cend();
 }
 
 SecondaryVariable const& SecondaryVariableCollection::get(
-    std::string const& external_name)
+    std::string const& external_name) const
 {
     auto const it = _map_external_to_internal.find(external_name);
 
diff --git a/ProcessLib/Output/SecondaryVariable.h b/ProcessLib/Output/SecondaryVariable.h
index 9ae67da8ae376d9ce510c6d55392199b7cc86ea9..d00eb112f81740ce335e10ba0272f410c7126dd1 100644
--- a/ProcessLib/Output/SecondaryVariable.h
+++ b/ProcessLib/Output/SecondaryVariable.h
@@ -130,10 +130,10 @@ public:
                               SecondaryVariableFunctions&& fcts);
 
     //! Returns the secondary variable with the given external name.
-    SecondaryVariable const& get(std::string const& external_name);
+    SecondaryVariable const& get(std::string const& external_name) const;
 
-    std::map<std::string, std::string>::const_iterator begin();
-    std::map<std::string, std::string>::const_iterator end();
+    std::map<std::string, std::string>::const_iterator begin() const;
+    std::map<std::string, std::string>::const_iterator end() const;
 
 private:
     //! Maps external variable names to internal ones.