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)
diff --git a/src/script/api/script_base.hpp b/src/script/api/script_base.hpp
--- a/src/script/api/script_base.hpp
+++ b/src/script/api/script_base.hpp
@@ -59,6 +59,7 @@ public:
 	 *   After all, it is a random function.
 	 * @param out How many times it should return true.
 	 * @param max Out of this many times.
+	 * @pre \a out is at most equal to \a max.
 	 * @return True if the chance worked out.
 	 */
 	static bool Chance(uint out, uint max);