Index: head/sys/kern/subr_blist.c
===================================================================
--- head/sys/kern/subr_blist.c
+++ head/sys/kern/subr_blist.c
@@ -764,6 +764,8 @@
 	/* Discard any candidates that appear before cursor. */
 	digit = (cursor / radix) & BLIST_META_MASK;
 	mask &= (u_daddr_t)-1 << digit;
+	if (mask == 0)
+		return (SWAPBLK_NONE);
 
 	/*
 	 * If the first try is for a block that includes the cursor, pre-undo