diff --git a/internal/transaction.go b/internal/transaction.go
index 582c07b..9006ae6 100644
--- a/internal/transaction.go
+++ b/internal/transaction.go
@@ -72,6 +72,8 @@
 	}
 	if readOnly {
 		req.Mode = pb.BeginTransactionRequest_READ_ONLY.Enum()
+	} else {
+		req.Mode = pb.BeginTransactionRequest_READ_WRITE.Enum()
 	}
 	if err := Call(c, "datastore_v3", "BeginTransaction", req, &t.transaction); err != nil {
 		return nil, err