diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index f022a4e..98c2d58 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1619,6 +1619,7 @@
         self.buffer.write(b)
         if self._line_buffering and (haslf or "\r" in s):
             self.flush()
+        self._set_decoded_chars('')
         self._snapshot = None
         if self._decoder:
             self._decoder.reset()
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index a725711..ebaf796 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2741,6 +2741,17 @@
         with self.maybeRaises(TypeError):
             t.read(42)
 
+    def test_issue25862(self):
+        # Assertion failures occurred in tell() after read() and write().
+        t = self.TextIOWrapper(self.BytesIO(b'test'), encoding='ascii')
+        t.read(1)
+        t.read()
+        t.tell()
+        t = self.TextIOWrapper(self.BytesIO(b'test'), encoding='ascii')
+        t.read(1)
+        t.write('x')
+        t.tell()
+
 
 class CTextIOWrapperTest(TextIOWrapperTest):