diff --git a/src/lib.rs b/src/lib.rs
index 9031eb6..3f19045 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -253,16 +253,14 @@ impl<'a> ops::Sub<&'a BitSet> for &'a BitSet {
     fn sub(self, rhs: &'a BitSet) -> BitSet {
         let mut set = self.clone();
 
-        set.nbits = 0;
-
         for (key, value) in &rhs.blocks {
             let block = set.blocks.entry(*key).or_insert(0);
 
             *block &= !value;
-
-            set.nbits += block.count_ones() as usize;
         }
 
+        set.nbits = set.blocks.values().map(|block| block.count_ones()).sum::<u32>() as usize;
+
         set
     }
 }
@@ -583,6 +581,21 @@ mod tests {
             i += 1;
         }
         assert_eq!(i, expected.len());
+        assert_eq!(set.len(), expected.len());
+
+        let a: BitSet = vec![1, 2, 3, 4, 5, 100, 101, 1001, 1002].into_iter().collect();
+        let b: BitSet = vec![1, 2, 3].into_iter().collect();
+
+        let set = &a - &b;
+
+        let mut i = 0;
+        let expected = [4, 5, 100, 101, 1001, 1002];
+        for x in &set {
+            assert!(expected.contains(&x));
+            i += 1;
+        }
+        assert_eq!(i, expected.len());
+        assert_eq!(set.len(), expected.len());
     }
 
     #[test]