diff --git a/Sources/FirebladeECS/UnorderedSparseSet.swift b/Sources/FirebladeECS/UnorderedSparseSet.swift
index 910cac4..d01d30f 100644
--- a/Sources/FirebladeECS/UnorderedSparseSet.swift
+++ b/Sources/FirebladeECS/UnorderedSparseSet.swift
@@ -12,7 +12,7 @@
 ///          an element from the sparse set.
 ///
 /// See <https://github.com/bombela/sparseset/blob/master/src/lib.rs> for a reference implementation.
-public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
+public final class UnorderedSparseSet<Element, Key: Hashable & Codable> {
     /// An index into the dense store.
     public typealias DenseIndex = Int
 
@@ -30,7 +30,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
     @usableFromInline var dense: DenseStore
     @usableFromInline var sparse: SparseStore
 
-    public init() {
+    public convenience init() {
         self.init(sparse: [:], dense: [])
     }
 
@@ -55,7 +55,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
     ///   - key: the key
     /// - Returns: true if new, false if replaced.
     @discardableResult
-    public mutating func insert(_ element: Element, at key: Key) -> Bool {
+    public func insert(_ element: Element, at key: Key) -> Bool {
         if let denseIndex = findIndex(at: key) {
             dense[denseIndex] = Entry(key: key, element: element)
             return false
@@ -63,7 +63,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
 
         let nIndex = dense.count
         dense.append(Entry(key: key, element: element))
-        sparse[key] = nIndex
+        sparse.updateValue(nIndex, forKey: key)
         return true
     }
 
@@ -86,7 +86,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
     /// - Parameter key: the key
     /// - Returns: removed value or nil if key not found.
     @discardableResult
-    public mutating func remove(at key: Key) -> Entry? {
+    public func remove(at key: Key) -> Entry? {
         guard let denseIndex = findIndex(at: key) else {
             return nil
         }
@@ -101,7 +101,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
     }
 
     @inlinable
-    public mutating func removeAll(keepingCapacity: Bool = false) {
+    public func removeAll(keepingCapacity: Bool = false) {
         sparse.removeAll(keepingCapacity: keepingCapacity)
         dense.removeAll(keepingCapacity: keepingCapacity)
     }
@@ -111,7 +111,7 @@ public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
     ///
     /// - Parameter denseIndex: the dense index
     /// - Returns: the element entry
-    private mutating func swapRemove(at denseIndex: Int) -> Entry {
+    private func swapRemove(at denseIndex: Int) -> Entry {
         dense.swapAt(denseIndex, dense.count - 1)
         return dense.removeLast()
     }
diff --git a/Tests/FirebladeECSTests/SparseSetTests.swift b/Tests/FirebladeECSTests/SparseSetTests.swift
index f02f4ec..5bf6d42 100644
--- a/Tests/FirebladeECSTests/SparseSetTests.swift
+++ b/Tests/FirebladeECSTests/SparseSetTests.swift
@@ -387,7 +387,7 @@ class SparseSetTests: XCTestCase {
 
     func testSparseSetDoubleRemove() {
         class AClass { }
-        var set = UnorderedSparseSet<AClass, Int>()
+        let set = UnorderedSparseSet<AClass, Int>()
         let a = AClass()
         let b = AClass()
         set.insert(a, at: 0)
@@ -471,7 +471,7 @@ class SparseSetTests: XCTestCase {
     }
 
     func testSparseSetReduce() {
-        var characters = UnorderedSparseSet<Character, Int>()
+        let characters = UnorderedSparseSet<Character, Int>()
 
         characters.insert("H", at: 4)
         characters.insert("e", at: 13)
@@ -497,7 +497,7 @@ class SparseSetTests: XCTestCase {
     }
 
     func testSubscript() {
-        var characters = UnorderedSparseSet<Character, Int>()
+        let characters = UnorderedSparseSet<Character, Int>()
 
         characters[4] = "H"
         characters[13] = "e"
@@ -528,7 +528,7 @@ class SparseSetTests: XCTestCase {
     }
 
     func testStartEndIndex() {
-        var set = UnorderedSparseSet<Character, Int>()
+        let set = UnorderedSparseSet<Character, Int>()
 
         set.insert("C", at: 33)
         set.insert("A", at: 11)
@@ -541,7 +541,7 @@ class SparseSetTests: XCTestCase {
 
     func testAlternativeKey() {
 
-        var set = UnorderedSparseSet<Character, String>()
+        let set = UnorderedSparseSet<Character, String>()
 
         set.insert("A", at: "a")
         set.insert("C", at: "c")