diff --git a/repo2docker/docker.py b/repo2docker/docker.py
index 8116ae8b..64f056ba 100644
--- a/repo2docker/docker.py
+++ b/repo2docker/docker.py
@@ -117,7 +117,7 @@ class DockerEngine(ContainerEngine):
             args += ["--tag", tag]
 
         if labels:
-            for k, v in labels:
+            for k, v in labels.items():
                 args += ["--label", f"{k}={v}"]
 
         if platform: