diff --git a/sys/sys/bitset.h b/sys/sys/bitset.h
--- a/sys/sys/bitset.h
+++ b/sys/sys/bitset.h
@@ -193,11 +193,13 @@
 
 #define	BIT_TEST_CLR_ATOMIC(_s, n, p)					\
 	(atomic_testandclear_long(					\
-	    &(p)->__bits[__bitset_word((_s), (n))], (n)) != 0)
+	    &(p)->__bits[__bitset_word((_s), (n))],			\
+	    (n) % _BITSET_BITS) != 0)
 
 #define	BIT_TEST_SET_ATOMIC(_s, n, p)					\
 	(atomic_testandset_long(					\
-	    &(p)->__bits[__bitset_word((_s), (n))], (n)) != 0)
+	    &(p)->__bits[__bitset_word((_s), (n))],			\
+	    (n) % _BITSET_BITS) != 0)
 
 /* Convenience functions catering special cases. */
 #define	BIT_AND_ATOMIC(_s, d, s) do {					\