diff --git a/src/storm-pars/api/region.h b/src/storm-pars/api/region.h
index 3665790ab..2cf39ddea 100644
--- a/src/storm-pars/api/region.h
+++ b/src/storm-pars/api/region.h
@@ -39,7 +39,7 @@ namespace storm {
         };
 
         template <typename ValueType>
-        std::vector<storm::storage::ParameterRegion<ValueType>> parseRegions(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> splittingThreshold = boost::none) {
+        std::vector<storm::storage::ParameterRegion<ValueType>> parseRegions(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none) {
             // If the given input string looks like a file (containing a dot and there exists a file with that name),
             // we try to parse it as a file, otherwise we assume it's a region string.
             if (inputString.find(".") != std::string::npos && std::ifstream(inputString).good()) {
@@ -50,12 +50,12 @@ namespace storm {
         }
 
         template <typename ValueType>
-        storm::storage::ParameterRegion<ValueType> createRegion(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> splittingThreshold= boost::none) {
+        storm::storage::ParameterRegion<ValueType> createRegion(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none) {
             return storm::parser::ParameterRegionParser<ValueType>().createRegion(inputString, consideredVariables, splittingThreshold);
         }
 
         template <typename ValueType>
-        std::vector<storm::storage::ParameterRegion<ValueType>> parseRegions(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> splittingThreshold= boost::none) {
+        std::vector<storm::storage::ParameterRegion<ValueType>> parseRegions(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> const& splittingThreshold = boost::none) {
             std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> modelParameters;
             if (model.isSparseModel()) {
                 auto const& sparseModel = dynamic_cast<storm::models::sparse::Model<ValueType> const&>(model);
@@ -69,7 +69,7 @@ namespace storm {
         }
 
         template <typename ValueType>
-        std::vector<storm::storage::ParameterRegion<ValueType>> createRegion(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> splittingThreshold= boost::none) {
+        std::vector<storm::storage::ParameterRegion<ValueType>> createRegion(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> const& splittingThreshold = boost::none) {
             std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> modelParameters;
             if (model.isSparseModel()) {
                 auto const& sparseModel = dynamic_cast<storm::models::sparse::Model<ValueType> const&>(model);
@@ -83,7 +83,7 @@ namespace storm {
         }
         
         template <typename ValueType>
-        storm::storage::ParameterRegion<ValueType> parseRegion(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> splittingThreshold= boost::none) {
+        storm::storage::ParameterRegion<ValueType> parseRegion(std::string const& inputString, std::set<typename storm::storage::ParameterRegion<ValueType>::VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none) {
             // Handle the "empty region" case
             if (inputString == "" && consideredVariables.empty()) {
                 return storm::storage::ParameterRegion<ValueType>();
@@ -95,7 +95,7 @@ namespace storm {
         }
         
         template <typename ValueType>
-        storm::storage::ParameterRegion<ValueType> parseRegion(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> splittingThreshold= boost::none) {
+        storm::storage::ParameterRegion<ValueType> parseRegion(std::string const& inputString, storm::models::ModelBase const& model, boost::optional<int> const& splittingThreshold = boost::none) {
             // Handle the "empty region" case
             if (inputString == "" && !model.hasParameters()) {
                 return storm::storage::ParameterRegion<ValueType>();
diff --git a/src/storm-pars/parser/ParameterRegionParser.cpp b/src/storm-pars/parser/ParameterRegionParser.cpp
index 4dcb95708..ca6dcd6c3 100644
--- a/src/storm-pars/parser/ParameterRegionParser.cpp
+++ b/src/storm-pars/parser/ParameterRegionParser.cpp
@@ -42,7 +42,7 @@ namespace storm {
         }
 
         template<typename ParametricType>
-        storm::storage::ParameterRegion<ParametricType> ParameterRegionParser<ParametricType>::parseRegion(std::string const& regionString, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold) {
+        storm::storage::ParameterRegion<ParametricType> ParameterRegionParser<ParametricType>::parseRegion(std::string const& regionString, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold) {
             Valuation lowerBoundaries;
             Valuation upperBoundaries;
             std::vector<std::string> parameterBoundaries;
@@ -66,7 +66,7 @@ namespace storm {
         }
 
         template<typename ParametricType>
-        storm::storage::ParameterRegion<ParametricType> ParameterRegionParser<ParametricType>::createRegion(std::string const& regionBound, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold) {
+        storm::storage::ParameterRegion<ParametricType> ParameterRegionParser<ParametricType>::createRegion(std::string const& regionBound, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold) {
             Valuation lowerBoundaries;
             Valuation upperBoundaries;
             std::vector<std::string> parameterBoundaries;
@@ -84,7 +84,7 @@ namespace storm {
         }
 
         template<typename ParametricType>
-        std::vector<storm::storage::ParameterRegion<ParametricType>> ParameterRegionParser<ParametricType>::parseMultipleRegions(std::string const& regionsString, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold) {
+        std::vector<storm::storage::ParameterRegion<ParametricType>> ParameterRegionParser<ParametricType>::parseMultipleRegions(std::string const& regionsString, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold) {
             std::vector<storm::storage::ParameterRegion<ParametricType>> result;
             std::vector<std::string> regionsStrVec;
             boost::split(regionsStrVec, regionsString, boost::is_any_of(";"));
@@ -97,7 +97,7 @@ namespace storm {
         }
 
         template<typename ParametricType>
-        std::vector<storm::storage::ParameterRegion<ParametricType>> ParameterRegionParser<ParametricType>::parseMultipleRegionsFromFile(std::string const& fileName, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold) {
+        std::vector<storm::storage::ParameterRegion<ParametricType>> ParameterRegionParser<ParametricType>::parseMultipleRegionsFromFile(std::string const& fileName, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold) {
      
             // Open file and initialize result.
             std::ifstream inputFileStream;
diff --git a/src/storm-pars/parser/ParameterRegionParser.h b/src/storm-pars/parser/ParameterRegionParser.h
index fb574248e..37b59e209 100644
--- a/src/storm-pars/parser/ParameterRegionParser.h
+++ b/src/storm-pars/parser/ParameterRegionParser.h
@@ -25,21 +25,21 @@ namespace storm {
              * Parse a single region from a string of the form "0.3<=p<=0.5,0.4<=q<=0.7".
              *
              */
-            static storm::storage::ParameterRegion<ParametricType> parseRegion(std::string const& regionString, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold = boost::none);
-            static storm::storage::ParameterRegion<ParametricType> createRegion(std::string const& regionBound, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold = boost::none);
+            static storm::storage::ParameterRegion<ParametricType> parseRegion(std::string const& regionString, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none);
+            static storm::storage::ParameterRegion<ParametricType> createRegion(std::string const& regionBound, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none);
 
             /*
              * Parse a vector of region from a string of the form "0.3<=p<=0.5,0.4<=q<=0.7;0.1<=p<=0.3,0.2<=q<=0.4".
              *
              */
-            static std::vector<storm::storage::ParameterRegion<ParametricType>> parseMultipleRegions(std::string const& regionsString, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold = boost::none);
+            static std::vector<storm::storage::ParameterRegion<ParametricType>> parseMultipleRegions(std::string const& regionsString, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none);
             
 
             /*
              * Parse multiple regions from a file
              *
              */
-            static std::vector<storm::storage::ParameterRegion<ParametricType>> parseMultipleRegionsFromFile(std::string const& fileName, std::set<VariableType> const& consideredVariables, boost::optional<int> splittingThreshold = boost::none);
+            static std::vector<storm::storage::ParameterRegion<ParametricType>> parseMultipleRegionsFromFile(std::string const& fileName, std::set<VariableType> const& consideredVariables, boost::optional<int> const& splittingThreshold = boost::none);
             
         };
     }