diff --git a/src/p_setup.c b/src/p_setup.c
index 899f5f618..408a52d6f 100644
--- a/src/p_setup.c
+++ b/src/p_setup.c
@@ -7688,7 +7688,7 @@ static void P_ConvertBinaryThingTypes(void)
 			break;
 		case FLOOR_SLOPE_THING:
 		case CEILING_SLOPE_THING:
-			mapthings[i].tid = mapthings[i].extrainfo;
+			mapthings[i].args[0] = mapthings[i].extrainfo;
 			break;
 		default:
 			break;