diff --git a/dataset/table.py b/dataset/table.py
index 45600ff..119d155 100644
--- a/dataset/table.py
+++ b/dataset/table.py
@@ -132,7 +132,8 @@ class Table(object):
         sync_row = {}
         for row in rows:
             # Only get non-existing columns.
-            for key in [k for k in row.keys() if k not in sync_row.keys()]:
+            sync_keys = list(sync_row.keys())
+            for key in [k for k in row.keys() if k not in sync_keys]:
                 # Get a sample of the new column(s) from the row.
                 sync_row[key] = row[key]
         self._sync_columns(sync_row, ensure, types=types)
@@ -197,7 +198,9 @@ class Table(object):
         columns = []
         for index, row in enumerate(rows):
             chunk.append(row)
-            columns = [col for col in row.keys() if col not in columns]
+            for col in row.keys():
+                if col not in columns:
+                    columns.append(col)
 
             # bindparam requires names to not conflict (cannot be "id" for id)
             for key in keys: