From d2e8d205b90f37c151c7ef1f85f7f404585c6051 Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Thu, 14 Jan 2016 19:09:34 +0000
Subject: [PATCH] Tracerestrict: Fix copying an unrestricted signal making an
 empty program.

---
 src/tracerestrict.cpp | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/tracerestrict.cpp b/src/tracerestrict.cpp
index caeb935acc..9100f1a8ae 100644
--- a/src/tracerestrict.cpp
+++ b/src/tracerestrict.cpp
@@ -1002,17 +1002,17 @@ CommandCost CmdProgramSignalTraceRestrictProgMgmt(TileIndex tile, DoCommandFlag
 	switch (type) {
 		case TRDCT_PROG_COPY: {
 			TraceRestrictRemoveProgramMapping(self);
-			TraceRestrictProgram *prog = GetTraceRestrictProgram(self, true);
-			if (!prog) {
-				// allocation failed
-				return CMD_ERROR;
-			}
 
 			TraceRestrictProgram *source_prog = GetTraceRestrictProgram(source, false);
-			if (source_prog) {
+			if (source_prog && !source_prog->items.empty()) {
+				TraceRestrictProgram *prog = GetTraceRestrictProgram(self, true);
+				if (!prog) {
+					// allocation failed
+					return CMD_ERROR;
+				}
 				prog->items = source_prog->items; // copy
+				prog->Validate();
 			}
-			prog->Validate();
 			break;
 		}