diff -r b382206aeb2b Lib/csv.py
--- a/Lib/csv.py	Fri Feb 03 11:08:45 2017 +0800
+++ b/Lib/csv.py	Sat Feb 04 14:10:34 2017 +0800
@@ -307,7 +307,7 @@
         charFrequency = {}
         modes = {}
         delims = {}
-        start, end = 0, min(chunkLength, len(data))
+        start, end = 0, chunkLength
         while start < len(data):
             iteration += 1
             for line in data[start:end]:
@@ -336,7 +336,7 @@
 
             # build a list of possible delimiters
             modeList = modes.items()
-            total = float(chunkLength * iteration)
+            total = float(min(chunkLength * iteration, len(data)))
             # (rows of consistent data) / (number of rows) = 100%
             consistency = 1.0
             # minimum consistency threshold