diff --git a/slate/slate.py b/slate/slate.py
index b1061e5..9b4af06 100644
--- a/slate/slate.py
+++ b/slate/slate.py
@@ -504,7 +504,7 @@ def insert_deep_dictionary(d: MutableMapping, t: tuple, value):
 
 def append_deep_dictionary(d: MutableMapping, t: tuple, value):
     if type(t) is tuple:
-        if len(t) is 1:
+        if len(t) == 1:
             if t[0] not in d:
                 d[t[0]] = []
             d[t[0]].append(value)