diff --git a/src/p_slopes.c b/src/p_slopes.c
index 659f8b33..4a8eadc5 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -557,7 +557,7 @@ static pslope_t *P_NewVertexSlope(INT16 tag1, INT16 tag2, INT16 tag3, UINT8 flag
 	for (i = 0; i < 3; i++) {
 		mt = ret->vertices[i];
 		if (!mt) // If a vertex wasn't found, it's game over. There's nothing you can do to recover (except maybe try and kill the slope instead - TODO?)
-			I_Error("Slope vertex %d (for linedef tag %d) not found.", i, tag1);
+			I_Error("Slope vertex %s (for linedef tag %d) not found.", sizeu1(i), tag1);
 		if (mt->extrainfo)
 			mt->z = mt->options;
 		else