diff --git a/Cargo.toml b/Cargo.toml
index 2e370ce..dc00021 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,3 +6,4 @@ description = "Multisets/bags"
 keywords = ["multiset","bag","data-structure","collection","count"]
 license = "MIT/Apache-2.0"
 authors = ["Jake Mitchell <jacob.d.mitchell@gmail.com>"]
+edition = "2015"
diff --git a/src/multiset.rs b/src/multiset.rs
index b93045d..d0dac07 100644
--- a/src/multiset.rs
+++ b/src/multiset.rs
@@ -14,6 +14,7 @@ use std::collections::HashMap;
 use std::fmt;
 use std::hash::Hash;
 use std::iter::{FromIterator, IntoIterator};
+use std::num::NonZero;
 use std::ops::{Add, Sub};
 
 /// A hash-based multiset.
@@ -22,7 +23,7 @@ pub struct HashMultiSet<K>
 where
     K: Eq + Hash,
 {
-    elem_counts: HashMap<K, usize>,
+    elem_counts: HashMap<K, NonZero<usize>>,
     size: usize,
 }
 
@@ -31,28 +32,19 @@ where
 /// This `struct` is created by the [`iter`] method on [`HashMultiSet`].
 #[derive(Clone)]
 pub struct Iter<'a, K: 'a> {
-    iter: hash_map::Iter<'a, K, usize>,
-    duplicate: Option<(&'a K, &'a usize)>,
-    duplicate_index: usize,
+    iter: hash_map::Iter<'a, K, NonZero<usize>>,
+    current: Option<(&'a K, NonZero<usize>)>,
 }
 
 impl<'a, K> Iterator for Iter<'a, K> {
     type Item = &'a K;
 
     fn next(&mut self) -> Option<&'a K> {
-        if self.duplicate.is_none() {
-            self.duplicate = self.iter.next();
-        }
-        if let Some((key, count)) = self.duplicate {
-            self.duplicate_index += 1;
-            if self.duplicate_index >= *count {
-                self.duplicate = None;
-                self.duplicate_index = 0;
-            }
-            Some(key)
-        } else {
-            None
+        let (key, count) = self.current.take().or_else(|| self.iter.next().map(|(key, &count)| (key, count)))?;
+        if let Some(new_count) = NonZero::new(count.get() - 1) {
+            self.current = Some((key, new_count));
         }
+        Some(key)
     }
 }
 
@@ -94,11 +86,10 @@ where
     /// }
     /// assert_eq!(3, multiset.iter().count());
     /// ```
-    pub fn iter(&self) -> Iter<K> {
+    pub fn iter(&self) -> Iter<'_, K> {
         Iter {
             iter: self.elem_counts.iter(),
-            duplicate: None,
-            duplicate_index: 0,
+            current: None,
         }
     }
 
@@ -133,10 +124,10 @@ where
     /// assert_eq!(set.contains(&1), true);
     /// assert_eq!(set.contains(&4), false);
     /// ```
-    pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
+    pub fn contains<Q>(&self, value: &Q) -> bool
     where
         K: Borrow<Q>,
-        Q: Hash + Eq,
+        Q: Hash + Eq + ?Sized,
     {
         self.elem_counts.contains_key(value)
     }
@@ -186,7 +177,7 @@ where
     /// assert!(distinct.contains(&2));
     /// assert!(!distinct.contains(&3));
     /// ```
-    pub fn distinct_elements(&self) -> Keys<K, usize> {
+    pub fn distinct_elements(&self) -> Keys<'_, K, NonZero<usize>> {
         self.elem_counts.keys()
     }
 
@@ -223,14 +214,15 @@ where
     /// assert_eq!(3, multiset.count_of(&5));
     /// ```
     pub fn insert_times(&mut self, val: K, n: usize) {
-        self.size += n;
+        self.size = self.size.checked_add(n).expect("count overflow");
+        let Some(n) = NonZero::new(n) else { return; };
         match self.elem_counts.entry(val) {
             Entry::Vacant(view) => {
                 view.insert(n);
             }
             Entry::Occupied(mut view) => {
                 let v = view.get_mut();
-                *v += n;
+                *v = v.checked_add(n.get()).expect("count overflow");
             }
         }
     }
@@ -280,15 +272,18 @@ where
     /// assert!(multiset.count_of(&5) == 0);
     /// ```
     pub fn remove_times(&mut self, val: &K, times: usize) -> usize {
-        if let Some(count) = self.elem_counts.get_mut(val) {
-            if *count > times {
-                *count -= times;
-                self.size -= times;
-                return times;
-            }
-            self.size -= *count;
+        let Some(actual_count) = self.elem_counts.get_mut(val) else { return 0; };
+        let result = if let Some(new_count) = actual_count.get().checked_sub(times).and_then(NonZero::new) {
+            *actual_count = new_count;
+            times
         }
-        self.elem_counts.remove(val).unwrap_or(0)
+        else {
+            let result = actual_count.get();
+            self.elem_counts.remove(val);
+            result
+        };
+        self.size -= result;
+        result
     }
 
     /// Remove all of an element from the multiset.
@@ -308,8 +303,8 @@ where
     /// assert!(multiset.len() == 0);
     /// ```
     pub fn remove_all(&mut self, val: &K) {
-        self.size -= self.elem_counts.get(val).unwrap_or(&0);
-        self.elem_counts.remove(val);
+        let count = self.elem_counts.remove(val).map(NonZero::get).unwrap_or_default();
+        self.size -= count;
     }
 
     /// Counts the occurrences of `val`.
@@ -328,7 +323,7 @@ where
     /// assert_eq!(1, multiset.count_of(&1));
     /// ```
     pub fn count_of(&self, val: &K) -> usize {
-        self.elem_counts.get(val).map_or(0, |x| *x)
+        self.elem_counts.get(val).map(|c| c.get()).unwrap_or_default()
     }
 }
 
@@ -358,7 +353,7 @@ where
     /// ```
     fn add(mut self, rhs: HashMultiSet<T>) -> HashMultiSet<T> {
         for (val, count) in rhs.elem_counts {
-            self.insert_times(val, count);
+            self.insert_times(val, count.get());
         }
         self
     }
@@ -391,7 +386,7 @@ where
     /// ```
     fn sub(mut self, rhs: HashMultiSet<T>) -> HashMultiSet<T> {
         for (val, count) in rhs.elem_counts {
-            self.remove_times(&val, count);
+            self.remove_times(&val, count.get());
         }
         self
     }
@@ -537,4 +532,11 @@ mod test_multiset {
         set.remove(&'d');
         assert_eq!(set.len(), 0);
     }
+
+    #[test]
+    fn test_insert_times_zero() {
+        let mut a = HashMultiSet::new();
+        a.insert_times(5, 0);
+        assert!(!a.contains(&5));
+    }
 }