diff --git a/src/command.cpp b/src/command.cpp
index 4b63357448..7bf1b8cba6 100644
--- a/src/command.cpp
+++ b/src/command.cpp
@@ -668,3 +668,14 @@ callb_err:
 	ClearStorageChanges(false);
 	return false;
 }
+
+
+CommandCost CommandCost::AddCost(CommandCost ret)
+{
+	this->AddCost(ret.cost);
+	if (this->success && !ret.success) {
+		this->message = ret.message;
+		this->success = false;
+	}
+	return *this;
+}
diff --git a/src/command_type.h b/src/command_type.h
index 2f3fe29475..da310fe4ea 100644
--- a/src/command_type.h
+++ b/src/command_type.h
@@ -49,15 +49,7 @@ public:
 	 * @param ret the command to add the cost of.
 	 * @return this class.
 	 */
-	CommandCost AddCost(CommandCost ret)
-	{
-		this->AddCost(ret.cost);
-		if (this->success && !ret.success) {
-			this->message = ret.message;
-			this->success = false;
-		}
-		return *this;
-	}
+	CommandCost AddCost(CommandCost ret);
 
 	/**
 	 * Adds the given cost to the cost of the command.