diff --git a/Lib/fontTools/ttLib/tables/otConverters.py b/Lib/fontTools/ttLib/tables/otConverters.py
index d214938bc7..b49c95d9f7 100644
--- a/Lib/fontTools/ttLib/tables/otConverters.py
+++ b/Lib/fontTools/ttLib/tables/otConverters.py
@@ -1859,7 +1859,7 @@ def getReadArray(reader, offSize):
             offsets = readArray(count + 1)
             items = []
             lastOffset = offsets.pop(0)
-            reader.readData(lastOffset)  # In case first offset is not 0
+            reader.readData(lastOffset - 1)  # In case first offset is not 1
 
             for offset in offsets:
                 assert lastOffset <= offset
@@ -1880,7 +1880,7 @@ def getReadArray(reader, offSize):
             def get_read_item():
                 reader_copy = reader.copy()
                 offset_pos = reader.pos
-                data_pos = offset_pos + (count + 1) * offSize
+                data_pos = offset_pos + (count + 1) * offSize - 1
                 readArray = getReadArray(reader_copy, offSize)
 
                 def read_item(i):
@@ -1922,7 +1922,7 @@ def write(self, writer, font, tableDict, values, repeatIndex=None):
             ]
 
         offsets = [len(item) for item in items]
-        offsets = [0] + list(accumulate(offsets))
+        offsets = list(accumulate(offsets, initial=1))
 
         lastOffset = offsets[-1]
         offSize = (