diff --git a/repo2docker/__main__.py b/repo2docker/__main__.py
index 9018f32c..09916169 100644
--- a/repo2docker/__main__.py
+++ b/repo2docker/__main__.py
@@ -263,11 +263,8 @@ def make_r2d(argv=None):
         r2d.appendix = args.appendix
 
     for l in args.labels:
-        if "=" in l:
-            key, val = l.split("=", 1)
-            r2d.labels[key] = val
-        else:
-            r2d.labels[l] = ""
+        key, _, val = l.partition("=")
+        r2d.labels[key] = val
 
     for a in args.build_args:
         key, _, val = a.partition("=")