Index: sys/vm/uma_core.c
===================================================================
--- sys/vm/uma_core.c
+++ sys/vm/uma_core.c
@@ -2312,16 +2312,18 @@
 		 * fragmentation.
 		 */
 		if (keg->uk_free > reserve) {
-			if (!LIST_EMPTY(&keg->uk_part_slab)) {
+			if (!LIST_EMPTY(&keg->uk_part_slab))
 				slab = LIST_FIRST(&keg->uk_part_slab);
-			} else {
+			else if (!LIST_EMPTY(&keg->uk_free_slab)) {
 				slab = LIST_FIRST(&keg->uk_free_slab);
 				LIST_REMOVE(slab, us_link);
 				LIST_INSERT_HEAD(&keg->uk_part_slab, slab,
 				    us_link);
 			}
-			MPASS(slab->us_keg == keg);
-			return (slab);
+			if (slab != NULL) {
+				MPASS(slab->us_keg == keg);
+				return (slab);
+			}
 		}
 
 		/*