diff --git a/toolz/itertoolz.py b/toolz/itertoolz.py
index 1ab54851..1115ac4e 100644
--- a/toolz/itertoolz.py
+++ b/toolz/itertoolz.py
@@ -732,7 +732,12 @@ def partition_all(n, seq):
         try:
             # If seq defines __len__, then
             # we can quickly calculate where no_pad starts
-            yield prev[:len(seq) % n]
+            end = len(seq) % n
+            if prev[end - 1] is no_pad or prev[end] is not no_pad:
+                raise LookupError(
+                    'The sequence passed to `parition_all` has invalid length'
+                )
+            yield prev[:end]
         except TypeError:
             # Get first index of no_pad without using .index()
             # https://github.com/pytoolz/toolz/issues/387
diff --git a/toolz/tests/test_itertoolz.py b/toolz/tests/test_itertoolz.py
index 27907b9e..b0da58a9 100644
--- a/toolz/tests/test_itertoolz.py
+++ b/toolz/tests/test_itertoolz.py
@@ -355,6 +355,20 @@ def __eq__(self, other):
     assert list(partition_all(4, [obj]*7)) == result
     assert list(partition_all(4, iter([obj]*7))) == result
 
+    # Test invalid __len__: https://github.com/pytoolz/toolz/issues/602
+    class ListWithBadLength(list):
+        def __init__(self, contents, off_by=1):
+            self.off_by = off_by
+            super().__init__(contents)
+
+        def __len__(self):
+            return super().__len__() + self.off_by
+
+    too_long_list = ListWithBadLength([1, 2], off_by=+1)
+    assert raises(LookupError, lambda: list(partition_all(5, too_long_list)))
+    too_short_list = ListWithBadLength([1, 2], off_by=-1)
+    assert raises(LookupError, lambda: list(partition_all(5, too_short_list)))
+
 
 def test_count():
     assert count((1, 2, 3)) == 3