--- a/sys/vfs/hammer2/hammer2_vfsops.c
+++ b/sys/vfs/hammer2/hammer2_vfsops.c
@@ -1017,7 +1031,12 @@
 		else
 			*label = '\0';		/* clean up trailing @ */
 
-		slice = label[-1];
+		/* DF-2622: do not read below devstr[] when the device
+		 * string is empty or a bare '@' (label == devstr). */
+		if (label > devstr)
+			slice = label[-1];
+		else
+			slice = 0;
 		switch(slice) {
 		case 'a':
 			label = "BOOT";