diff --git a/internal/mapset/immutable.go b/internal/mapset/immutable.go
index 73b54a1..d80f8db 100644
--- a/internal/mapset/immutable.go
+++ b/internal/mapset/immutable.go
@@ -36,9 +36,8 @@ func (h ImmutableMapSet[T]) Len() int {
 }
 
 // Equal returns whether the same items exist in both h and o
-func (h ImmutableMapSet[T]) Equal(o ImmutableMapSet[T]) bool {
-	om := MapSet[T](o)
-	return MapSet[T](h).Equal(&om)
+func (h ImmutableMapSet[T]) Equal(o Container[T]) bool {
+	return MapSet[T](h).Equal(o)
 }
 
 // MarshalJSON serializes a MapSet as a JSON array. Elements are ordered lexicographically by their marshaled value.
diff --git a/internal/mapset/immutable_test.go b/internal/mapset/immutable_test.go
index 663b8d6..b58b716 100644
--- a/internal/mapset/immutable_test.go
+++ b/internal/mapset/immutable_test.go
@@ -60,6 +60,16 @@ func TestImmutableHashSet(t *testing.T) {
 		testutil.Equals(t, s1.Equal(s3), false)
 	})
 
+	t.Run("equality of MapSet and ImmutableMapSet", func(t *testing.T) {
+		s1 := FromItems(1, 2, 3, 4)
+		s2 := Immutable(1, 2, 3, 4)
+		s3 := Immutable(1, 2, 3)
+		testutil.Equals(t, s1.Equal(s2), true)
+		testutil.Equals(t, s2.Equal(s1), true)
+		testutil.Equals(t, s1.Equal(s3), false)
+		testutil.Equals(t, s3.Equal(s1), false)
+	})
+
 	t.Run("iterate", func(t *testing.T) {
 		s1 := Immutable(1, 2, 3)
 
diff --git a/internal/mapset/mapset.go b/internal/mapset/mapset.go
index 8807064..40ed608 100644
--- a/internal/mapset/mapset.go
+++ b/internal/mapset/mapset.go
@@ -77,6 +77,7 @@ func (h MapSet[T]) Contains(item T) bool {
 
 type Container[T comparable] interface {
 	Contains(T) bool
+	Len() int
 }
 
 // Intersects returns whether any items in this set exist in o
@@ -112,8 +113,8 @@ func (h MapSet[T]) Len() int {
 }
 
 // Equal returns whether the same items exist in both h and o
-func (h MapSet[T]) Equal(o *MapSet[T]) bool {
-	if len(h.m) != len(o.m) {
+func (h MapSet[T]) Equal(o Container[T]) bool {
+	if len(h.m) != o.Len() {
 		return false
 	}