diff --git a/src/env_universal_common.cpp b/src/env_universal_common.cpp
index 7388b4825..7fd8bf1e9 100644
--- a/src/env_universal_common.cpp
+++ b/src/env_universal_common.cpp
@@ -263,7 +263,7 @@ maybe_t<env_var_t::env_var_flags_t> env_universal_t::get_flags(const wcstring &n
     return none();
 }
 
-void env_universal_t::set_internal(const wcstring &key, env_var_t var, bool overwrite) {
+void env_universal_t::set_internal(const wcstring &key, const env_var_t &var, bool overwrite) {
     ASSERT_IS_LOCKED(lock);
     if (!overwrite && this->modified.find(key) != this->modified.end()) {
         // This value has been modified and we're not overwriting it. Skip it.
diff --git a/src/env_universal_common.h b/src/env_universal_common.h
index d418db7cc..6e055db68 100644
--- a/src/env_universal_common.h
+++ b/src/env_universal_common.h
@@ -56,7 +56,7 @@ class env_universal_t {
     bool load_from_path(const wcstring &path, callback_data_list_t &callbacks);
     void load_from_fd(int fd, callback_data_list_t &callbacks);
 
-    void set_internal(const wcstring &key, env_var_t var, bool overwrite);
+    void set_internal(const wcstring &key, const env_var_t &var, bool overwrite);
     bool remove_internal(const wcstring &name);
 
     // Functions concerned with saving.