diff --git a/src/slope_anchors.c b/src/slope_anchors.c
index d40fc7318..18a9ac46e 100644
--- a/src/slope_anchors.c
+++ b/src/slope_anchors.c
@@ -473,7 +473,7 @@ static void P_SetupAnchoredSlopes (void) {
 
 			if (plane == 0)
 			{
-				CONS_Alert(CONS_WARNING, "Slope anchor linedef %u has no planes set.\n", i);
+				CONS_Alert(CONS_WARNING, "Slope anchor linedef %s has no planes set.\n", sizeu1(i));
 				continue;
 			}