diff --git a/src/ResearchTree/ResearchTree.cpp b/src/ResearchTree/ResearchTree.cpp
index d5b2f64..bda6279 100644
--- a/src/ResearchTree/ResearchTree.cpp
+++ b/src/ResearchTree/ResearchTree.cpp
@@ -384,7 +384,7 @@ void ResearchTree::readEngineUnlocks(Containers::ArrayView<Int> engines, Contain
         return;
     }
 
-    for(Int& engine : engines) {
+    for(Int engine : engines) {
         if(_engineNodes.find(engine) != _engineNodes.end()) {
             _engineNodes.at(engine).setState(Node::State::Unlocked);
         }
@@ -394,7 +394,7 @@ void ResearchTree::readEngineUnlocks(Containers::ArrayView<Int> engines, Contain
         return;
     }
 
-    for(Int& gear : gears) {
+    for(Int gear : gears) {
         if(_engineNodes.find(gear) != _engineNodes.end()) {
             _engineNodes.at(gear).setState(Node::State::Unlocked);
         }