diff --git a/sidecar/sidecar.py b/sidecar/sidecar.py
index f6b9d9a..8e8916d 100644
--- a/sidecar/sidecar.py
+++ b/sidecar/sidecar.py
@@ -33,7 +33,7 @@ class Sidecar(Output):
 
     @observe('ref')
     def _validate_ref_anchor(self, *args):
-        if self.ref.anchor == 'right':
+        if self.ref and self.ref.anchor == 'right':
             warnings.warn(
                 "`ref` cannot be set when `ref.anchor == 'right'`. "
                 "Proceeding with `ref = None`.", UserWarning