diff --git a/implementations/python/preserves/binary.py b/implementations/python/preserves/binary.py
index 5704b4c..0cf2a7b 100644
--- a/implementations/python/preserves/binary.py
+++ b/implementations/python/preserves/binary.py
@@ -200,6 +200,8 @@ class Encoder(BinaryCodec):
             try:
                 i = iter(v)
             except TypeError:
+                i = None
+            if i is None:
                 raise TypeError('Cannot preserves-encode: ' + repr(v))
             self.encodevalues(5, i)
 
diff --git a/implementations/python/preserves/text.py b/implementations/python/preserves/text.py
index c36a739..bb368bb 100644
--- a/implementations/python/preserves/text.py
+++ b/implementations/python/preserves/text.py
@@ -389,6 +389,8 @@ class Formatter(TextCodec):
             try:
                 i = iter(v)
             except TypeError:
+                i = None
+            if i is None:
                 raise TypeError('Cannot preserves-format: ' + repr(v))
             self.write_seq('[', ']', i)