diff --git a/src/script/api/script_town.cpp b/src/script/api/script_town.cpp
--- a/src/script/api/script_town.cpp
+++ b/src/script/api/script_town.cpp
@@ -154,12 +154,12 @@
 	}
 }
 
-/* static */ bool ScriptTown::SetGrowthRate(TownID town_id, uint16 days_between_town_growth)
+/* static */ bool ScriptTown::SetGrowthRate(TownID town_id, uint32 days_between_town_growth)
 {
 	days_between_town_growth = days_between_town_growth * DAY_TICKS / TOWN_GROWTH_TICKS;
 
 	EnforcePrecondition(false, IsValidTown(town_id));
-	EnforcePrecondition(false, (days_between_town_growth & TOWN_GROW_RATE_CUSTOM) == 0);
+	EnforcePrecondition(false, days_between_town_growth < TOWN_GROW_RATE_CUSTOM);
 
 	return ScriptObject::DoCommand(::Town::Get(town_id)->xy, town_id, days_between_town_growth, CMD_TOWN_GROWTH_RATE);
 }
diff --git a/src/script/api/script_town.hpp b/src/script/api/script_town.hpp
--- a/src/script/api/script_town.hpp
+++ b/src/script/api/script_town.hpp
@@ -251,12 +251,13 @@ public:
 	 * @param town_id The index of the town.
 	 * @param days_between_town_growth The amount of days between town growth.
 	 * @pre IsValidTown(town_id).
+	 * @pre days_between_town_growth <= 30000.
 	 * @return True if the action succeeded.
 	 * @note Even when setting a growth rate, towns only grow when the conditions for growth (SetCargoCoal) are met,
 	 *       and the game settings (economy.town_growth_rate) allow town growth at all.
 	 * @api -ai
 	 */
-	static bool SetGrowthRate(TownID town_id, uint16 days_between_town_growth);
+	static bool SetGrowthRate(TownID town_id, uint32 days_between_town_growth);
 
 	/**
 	 * Get the amount of days between town growth.