diff --git a/hammer/hammer.go b/hammer/hammer.go
index 013fd08..31f99b8 100644
--- a/hammer/hammer.go
+++ b/hammer/hammer.go
@@ -301,7 +301,10 @@ func genLeaf(n uint64, minLeafSize int) []byte {
 func newLeafGenerator(n uint64, minLeafSize int) func() []byte {
 	const dupChance = 0.1
 	nextLeaf := genLeaf(n, minLeafSize)
+	var safe sync.Mutex
 	return func() []byte {
+		safe.Lock()
+		defer safe.Unlock()
 		if rand.Float64() <= dupChance {
 			// This one will actually be unique, but the next iteration will
 			// duplicate it. In future, this duplication could be randomly