diff --git a/src/emsdevice.cpp b/src/emsdevice.cpp
index de9a88eb7..715484bb0 100644
--- a/src/emsdevice.cpp
+++ b/src/emsdevice.cpp
@@ -489,7 +489,7 @@ bool EMSdevice::is_readable(const void * value_p) const {
 }
 
 // check if value/command is readonly
-bool EMSdevice::is_readonly(const std::string cmd, const int8_t id) const {
+bool EMSdevice::is_readonly(const std::string & cmd, const int8_t id) const {
     uint8_t tag = id > 0 ? DeviceValueTAG::TAG_HC1 + id - 1 : DeviceValueTAG::TAG_NONE;
     for (const auto & dv : devicevalues_) {
         // check command name and tag, id -1 is default hc and only checks name
diff --git a/src/emsdevice.h b/src/emsdevice.h
index d95f92862..a521a042c 100644
--- a/src/emsdevice.h
+++ b/src/emsdevice.h
@@ -243,7 +243,7 @@ class EMSdevice {
     void read_command(const uint16_t type_id, uint8_t offset = 0, uint8_t length = 0) const;
 
     bool is_readable(const void * value_p) const;
-    bool is_readonly(const std::string cmd, const int8_t id) const;
+    bool is_readonly(const std::string & cmd, const int8_t id) const;
     bool has_command(const void * value_p) const;
     void publish_value(void * value_p) const;
     void publish_all_values();