diff --git a/mvt.cpp b/mvt.cpp
index e802d4ed..779d8214 100644
--- a/mvt.cpp
+++ b/mvt.cpp
@@ -583,7 +583,7 @@ std::string mvt_value::toString() const {
 	}
 }
 
-void mvt_layer::tag(mvt_feature &feature, std::string key, mvt_value value) {
+void mvt_layer::tag(mvt_feature &feature, std::string const &key, mvt_value const &value) {
 	size_t ko, vo;
 
 	// initialize lazily the first time anyone tags an attribute
diff --git a/mvt.hpp b/mvt.hpp
index b0f53413..2f25ddc9 100644
--- a/mvt.hpp
+++ b/mvt.hpp
@@ -143,7 +143,7 @@ struct mvt_layer {
 	long long extent = 0;
 
 	// Add a key-value pair to a feature, using this layer's constant pool
-	void tag(mvt_feature &feature, std::string key, mvt_value value);
+	void tag(mvt_feature &feature, std::string const &key, mvt_value const &value);
 
 	// For tracking the key-value constants already used in this layer
 	std::unordered_map<std::string, size_t> key_map{};