diff --git a/README.md b/README.md
index fce0ea3..73c4b8d 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,3 @@
 # mdb
 
 An in-process, in-memory database for Go.
-
-## TO DO
-
-* BTreeIndex:
-  * Panic if insert or update replaces an item
diff --git a/btreeindex.go b/btreeindex.go
index cb24a95..d2a4f36 100644
--- a/btreeindex.go
+++ b/btreeindex.go
@@ -118,8 +118,9 @@ func (t *BTreeIndex[T]) Len() int {
 func (t *BTreeIndex[T]) insert(item *T) {
 	if t.include == nil || t.include(item) {
 		t.modify(func(bt *btree.BTreeG[*T]) {
-			// TODO: Panic if replaces.
-			bt.ReplaceOrInsert(item)
+			if _, ok := bt.ReplaceOrInsert(item); ok {
+				panic("Insert replaced an item.")
+			}
 		})
 	}
 }
@@ -130,8 +131,9 @@ func (t *BTreeIndex[T]) update(old, new *T) {
 			bt.Delete(old)
 		}
 		if t.include == nil || t.include(new) {
-			// TODO: Panic if replaces.
-			bt.ReplaceOrInsert(new)
+			if _, ok := bt.ReplaceOrInsert(new); ok {
+				panic("Update insert replaced an item.")
+			}
 		}
 	})
 }
diff --git a/shipping_test.go b/shipping_test.go
index 6a0077c..7cfd7f7 100644
--- a/shipping_test.go
+++ b/shipping_test.go
@@ -109,7 +109,7 @@ func TestShipping(t *testing.T) {
 		for i := 0; i < 64; i++ {
 			go func() {
 				defer updateWG.Done()
-				for j := 0; j < 1024; j++ {
+				for j := 0; j < 4096; j++ {
 					time.Sleep(sleepTimeout)
 					db.RandAction()
 				}