diff --git a/advtrains_interlocking/signal_api.lua b/advtrains_interlocking/signal_api.lua
index 6ca6a90..a44eda6 100644
--- a/advtrains_interlocking/signal_api.lua
+++ b/advtrains_interlocking/signal_api.lua
@@ -186,7 +186,7 @@ local function convert_aspect_if_necessary(asp)
 		else
 			newasp.main = 0
 		end
-		if asp.dst.free then
+		if asp.dst and asp.dst.free then
 			newasp.dst = asp.dst.speed
 		else
 			newasp.dst = 0