diff --git a/src/display.cpp b/src/display.cpp
index ddb00a2f3b30..f34e11ae9858 100644
--- a/src/display.cpp
+++ b/src/display.cpp
@@ -569,7 +569,7 @@ void display::change_map(const gamemap* m)
 	builder_->change_map(m);
 }
 
-void display::change_units(unit_map* umap)
+void display::change_units(const unit_map* umap)
 {
 	units_ = umap;
 }
diff --git a/src/display.hpp b/src/display.hpp
index 9c2139ffa40e..eb9306cd3c33 100644
--- a/src/display.hpp
+++ b/src/display.hpp
@@ -155,7 +155,7 @@ class display
 
 	void change_map(const gamemap* m);
 	void change_teams(const std::vector<team>* teams);
-	void change_units(unit_map* units);
+	void change_units(const unit_map* units);
 
 	static Uint32 rgb(Uint8 red, Uint8 green, Uint8 blue)
 		{ return 0xFF000000 | (red << 16) | (green << 8) | blue; }