diff --git a/map.cc b/map.cc
index 7b1842c..563a20c 100644
--- a/map.cc
+++ b/map.cc
@@ -4,10 +4,12 @@
 #include "agent.h"
 #include "map.h"
 
-void
+bool
 tile::on_agent_enter(class agent *a)
 {
+	if (agent) return false;
 	agent = a;
+	return true;
 }
 
 void
diff --git a/map.h b/map.h
index 369377a..ea5f4d5 100644
--- a/map.h
+++ b/map.h
@@ -8,7 +8,7 @@ class tile {
 public:
 	class agent *agent;
 
-	virtual void on_agent_enter(class agent *);
+	virtual bool on_agent_enter(class agent *);
 	virtual void on_agent_leave(class agent *);
 
 	virtual void on_tick(void);