diff --git a/roaring/roaring.go b/roaring/roaring.go
index 3cccf3af1..aa42fb380 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -4617,8 +4617,8 @@ func compareArrayArray(a1, a2 []uint16) error {
 // an error describing any difference it finds. This is mostly intended
 // for use in tests that expect equality.
 func (c *Container) BitwiseCompare(c2 *Container) error {
-	if c.N() != c2.N() {
-		return errors.New("containers are different lengths")
+	if cn, c2n := c.N(), c2.N(); cn != c2n {
+		return errors.Errorf("containers are different lengths: %d, %d", cn, c2n)
 	}
 	if c.N() == 0 {
 		return nil
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
index 4cf0b48d6..e52a29b23 100644
--- a/roaring/roaring_internal_test.go
+++ b/roaring/roaring_internal_test.go
@@ -4391,12 +4391,18 @@ func TestUnionRunRunInPlaceBitwiseCompare(t *testing.T) {
 	for _, a := range runs {
 		for _, b := range runs {
 			t.Run(a.name+"-"+b.name, func(t *testing.T) {
-				arun := doContainer(containerRun, a.run).Thaw()
-				brun := doContainer(containerRun, b.run).Thaw()
+				arun := doContainer(containerRun, a.run)
+				brun := doContainer(containerRun, b.run)
 
 				out1 := unionBitmapRunInPlace(arun.runToBitmap(), brun)
 				out2 := unionRunRunInPlace(arun, brun)
 
+				// Because these containers have been modified using in-place
+				// methods, we need to repair them before comparison to ensure
+				// that their `n` value is updated.
+				out1.Repair()
+				out2.Repair()
+
 				err := out1.BitwiseCompare(out2.runToBitmap())
 				if err != nil {
 					t.Fatal(err)