1 files changed, 3 insertions, 0 deletions
diff --git a/Lib/gzip.py b/Lib/gzip.py
index cf8b675..177f908 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -401,6 +401,9 @@ class GzipFile(_compression.BaseStream):
 
     def seek(self, offset, whence=io.SEEK_SET):
         if self.mode == WRITE:
+            self._check_not_closed()
+            # Flush buffer to ensure validity of self.offset
+            self._buffer.flush()
             if whence != io.SEEK_SET:
                 if whence == io.SEEK_CUR:
                     offset = self.offset + offset