diff --git a/implementations/python/preserves/binary.py b/implementations/python/preserves/binary.py
index 0cf2a7b..25745ad 100644
--- a/implementations/python/preserves/binary.py
+++ b/implementations/python/preserves/binary.py
@@ -202,8 +202,12 @@ class Encoder(BinaryCodec):
             except TypeError:
                 i = None
             if i is None:
-                raise TypeError('Cannot preserves-encode: ' + repr(v))
-            self.encodevalues(5, i)
+                self.cannot_encode(v)
+            else:
+                self.encodevalues(5, i)
+
+    def cannot_encode(self, v):
+        raise TypeError('Cannot preserves-encode: ' + repr(v))
 
 def encode(v, **kwargs):
     e = Encoder(**kwargs)
diff --git a/implementations/python/preserves/text.py b/implementations/python/preserves/text.py
index bb368bb..8b1ddfe 100644
--- a/implementations/python/preserves/text.py
+++ b/implementations/python/preserves/text.py
@@ -391,8 +391,12 @@ class Formatter(TextCodec):
             except TypeError:
                 i = None
             if i is None:
-                raise TypeError('Cannot preserves-format: ' + repr(v))
-            self.write_seq('[', ']', i)
+                self.cannot_format(v)
+            else:
+                self.write_seq('[', ']', i)
+
+    def cannot_format(self, v):
+        raise TypeError('Cannot preserves-format: ' + repr(v))
 
 def stringify(v, **kwargs):
     e = Formatter(**kwargs)