diff --git a/code/fgame/characterstate.cpp b/code/fgame/characterstate.cpp
index c280ad93..e0c672ff 100644
--- a/code/fgame/characterstate.cpp
+++ b/code/fgame/characterstate.cpp
@@ -77,7 +77,7 @@ Conditional::Conditional()
 
 Expression::Expression() {}
 
-Expression::Expression(Expression& exp)
+Expression::Expression(const Expression& exp)
 
 {
     int i;
diff --git a/code/fgame/characterstate.h b/code/fgame/characterstate.h
index 83d43b16..afc4d604 100644
--- a/code/fgame/characterstate.h
+++ b/code/fgame/characterstate.h
@@ -184,7 +184,7 @@ private:
 
 public:
     Expression();
-    Expression(Expression& exp);
+    Expression(const Expression& exp);
     Expression(Script& script, State& state);
 
     void operator=(const Expression& exp);