diff --git a/src/script/api/script_base.cpp b/src/script/api/script_base.cpp
--- a/src/script/api/script_base.cpp
+++ b/src/script/api/script_base.cpp
@@ -11,6 +11,7 @@
 
 #include "../../stdafx.h"
 #include "script_base.hpp"
+#include "script_error.hpp"
 #include "../../network/network.h"
 #include "../../core/random_func.hpp"
 
@@ -42,7 +43,8 @@
 
 /* static */ bool ScriptBase::Chance(uint out, uint max)
 {
-	return (uint16)Rand() <= (uint16)((65536 * out) / max);
+	EnforcePrecondition(false, out <= max);
+	return (uint16)Rand() <= (uint16)((65535 * out) / max);
 }
 
 /* static */ bool ScriptBase::ChanceItem(int unused_param, uint out, uint max)