diff --git a/ProcessLib/Output/SecondaryVariable.cpp b/ProcessLib/Output/SecondaryVariable.cpp
index 3dc19956745f3f8117b3e1a76aad707f5e96d43d..604b152d1d83dde9ff262114cbd342b2cff4ebcb 100644
--- a/ProcessLib/Output/SecondaryVariable.cpp
+++ b/ProcessLib/Output/SecondaryVariable.cpp
@@ -38,6 +38,18 @@ void SecondaryVariableCollection::addSecondaryVariable(
     }
 }
 
+std::map<std::string, std::string>::const_iterator
+SecondaryVariableCollection::begin()
+{
+    return _map_external_to_internal.cbegin();
+}
+
+std::map<std::string, std::string>::const_iterator
+SecondaryVariableCollection::end()
+{
+    return _map_external_to_internal.cend();
+}
+
 SecondaryVariable const& SecondaryVariableCollection::get(
     std::string const& external_name)
 {
@@ -46,7 +58,7 @@ SecondaryVariable const& SecondaryVariableCollection::get(
     if (it == _map_external_to_internal.cend())
     {
         OGS_FATAL(
-            "A secondary variable with external name `%s' has not been set up.",
+            "A secondary variable with external name '%s' has not been set up.",
             external_name.c_str());
     }
 
@@ -56,7 +68,7 @@ SecondaryVariable const& SecondaryVariableCollection::get(
     if (it2 == _configured_secondary_variables.end())
     {
         OGS_FATAL(
-            "A secondary variable with internal name `%s' has not been set up.",
+            "A secondary variable with internal name '%s' has not been set up.",
             internal_name.c_str());
     }
 
diff --git a/ProcessLib/Output/SecondaryVariable.h b/ProcessLib/Output/SecondaryVariable.h
index 0b268ff73fc094c008597f92dd162c084fd70428..9ae67da8ae376d9ce510c6d55392199b7cc86ea9 100644
--- a/ProcessLib/Output/SecondaryVariable.h
+++ b/ProcessLib/Output/SecondaryVariable.h
@@ -132,6 +132,9 @@ public:
     //! Returns the secondary variable with the given external name.
     SecondaryVariable const& get(std::string const& external_name);
 
+    std::map<std::string, std::string>::const_iterator begin();
+    std::map<std::string, std::string>::const_iterator end();
+
 private:
     //! Maps external variable names to internal ones.
     //! The external variable names are used, e.g., for output.