diff --git a/src/calibre/ebooks/mobi/utils.py b/src/calibre/ebooks/mobi/utils.py
index 3b8ce61ba8..0d55a8d4bd 100644
--- a/src/calibre/ebooks/mobi/utils.py
+++ b/src/calibre/ebooks/mobi/utils.py
@@ -577,7 +577,7 @@ class CNCX(object): # {{{
                 pad = 0xfbf8 - buf.tell()
                 buf.write(b'\0' * pad)
                 self.records.append(buf.getvalue())
-                buf.truncate(0)
+                buf.seek(0), buf.truncate(0)
                 offset = len(self.records) * 0x10000
             buf.write(raw)
             self.strings[key] = offset
diff --git a/src/calibre/ebooks/mobi/writer2/indexer.py b/src/calibre/ebooks/mobi/writer2/indexer.py
index be926a80a0..183697a1b4 100644
--- a/src/calibre/ebooks/mobi/writer2/indexer.py
+++ b/src/calibre/ebooks/mobi/writer2/indexer.py
@@ -500,14 +500,14 @@ class Indexer(object): # {{{
 
         # Write offsets to index entries as an IDXT block
         idxt_block = b'IDXT'
-        buf.truncate(0)
+        buf.seek(0), buf.truncate(0)
         for offset in offsets:
             buf.write(pack(b'>H', header_length+offset))
         idxt_block = align_block(idxt_block + buf.getvalue())
         body = index_block + idxt_block
 
         header = b'INDX'
-        buf.truncate(0)
+        buf.seek(0), buf.truncate(0)
         buf.write(pack(b'>I', header_length))
         buf.write(b'\0'*4) # Unknown
         buf.write(pack(b'>I', 1)) # Header type? Or index record number?