diff --git a/src/storage/gspn/GSPN.cpp b/src/storage/gspn/GSPN.cpp
index 0ea8deeb9..dfb9d0f57 100644
--- a/src/storage/gspn/GSPN.cpp
+++ b/src/storage/gspn/GSPN.cpp
@@ -57,7 +57,7 @@ namespace storm {
             return std::make_pair<bool, std::shared_ptr<storm::gspn::TimedTransition<GSPN::RateType>> const>(false, nullptr);
         }
 
-        std::pair<bool, std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const> GSPN::getImmediateTransition(std::string id) const {
+        std::pair<bool, std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const> GSPN::getImmediateTransition(std::string const& id) const {
             for (auto& trans : immediateTransitions) {
                 if (id.compare(trans->getName()) == 0) {
                     return std::make_pair<bool, std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const>(true, static_cast<std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const>(trans));
diff --git a/src/storage/gspn/GSPN.h b/src/storage/gspn/GSPN.h
index 3217af882..d81f93ad6 100644
--- a/src/storage/gspn/GSPN.h
+++ b/src/storage/gspn/GSPN.h
@@ -101,7 +101,7 @@ namespace storm {
              *         If the first element is true, then the second element is the wanted transition.
              *         If the first element is false, then the second element is the nullptr.
              */
-            std::pair<bool, std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const> getImmediateTransition(std::string id) const;
+            std::pair<bool, std::shared_ptr<storm::gspn::ImmediateTransition<GSPN::WeightType>> const> getImmediateTransition(std::string const& id) const;
 
             /*!
              * Returns the transition with the corresponding id.