diff --git a/src/lib.rs b/src/lib.rs
index 1ca026e..9031eb6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -258,7 +258,7 @@ impl<'a> ops::Sub<&'a BitSet> for &'a BitSet {
         for (key, value) in &rhs.blocks {
             let block = set.blocks.entry(*key).or_insert(0);
 
-            *block = !value & *block;
+            *block &= !value;
 
             set.nbits += block.count_ones() as usize;
         }