diff --git a/zfs-backup.py b/zfs-backup.py
index 7636f16..6469a78 100755
--- a/zfs-backup.py
+++ b/zfs-backup.py
@@ -257,7 +257,7 @@ class ZFS:
             raise ValueError(SAME_DATASET)
         print(f"  {old_snap} -> {new_snap}")
         sender = Popen(
-            [config.ZFS, "send", "-R", "-I", old_snap, new_snap], stdout=PIPE
+            [config.ZFS, "send", "-R", "-I", str(old_snap), str(new_snap)], stdout=PIPE
         )
         return sender