diff --git a/bedstead.c b/bedstead.c
index 275028d..5231513 100644
--- a/bedstead.c
+++ b/bedstead.c
@@ -1491,7 +1491,7 @@ dopalt(struct glyph const *g)
 	 * characters get an advance width of three pixels.
 	 */
 	for (i = 0; i < YSIZE; i++)
-		cols |= g->data[i];
+		cols |= g->data[i] & ((1 << XSIZE) - 1);
 	if (cols == 0)
 		dh = 3 - XSIZE;
 	else {