diff --git a/common/include/villas/kernel/pci.hpp b/common/include/villas/kernel/pci.hpp
index caa914a54..d22466cc2 100644
--- a/common/include/villas/kernel/pci.hpp
+++ b/common/include/villas/kernel/pci.hpp
@@ -118,7 +118,7 @@ private:
 	villas::Logger log;
 
 protected:
-	std::fstream openSysFs(const std::string subPath, std::ios_base::openmode mode = std::ios_base::in | std::ios_base::out) const;
+	std::fstream openSysFs(const std::string &subPath, std::ios_base::openmode mode = std::ios_base::in | std::ios_base::out) const;
 };
 
 class DeviceList : public std::list<std::shared_ptr<Device>> {
diff --git a/common/lib/kernel/pci.cpp b/common/lib/kernel/pci.cpp
index 9199f0dd4..42db0bd19 100644
--- a/common/lib/kernel/pci.cpp
+++ b/common/lib/kernel/pci.cpp
@@ -446,7 +446,7 @@ int Device::getIommuGroup() const
 	return atoi(group);
 }
 
-std::fstream Device::openSysFs(const std::string subPath, std::ios_base::openmode mode) const
+std::fstream Device::openSysFs(const std::string &subPath, std::ios_base::openmode mode) const
 {
 	std::fstream file;