# HG changeset patch
# Parent ffb58df8f9eac67c675e15463cc4f2843d8d9d74
diff --git a/Lib/gzip.py b/Lib/gzip.py
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -144,8 +144,10 @@
         if fileobj is None:
             fileobj = self.myfileobj = builtins.open(filename, mode or 'rb')
         if filename is None:
-            if hasattr(fileobj, 'name'): filename = fileobj.name
-            else: filename = ''
+            if hasattr(fileobj, 'name') and isinstance(fileobj.name, str):
+                filename = fileobj.name
+            else:
+                filename = ''
         if mode is None:
             if hasattr(fileobj, 'mode'): mode = fileobj.mode
             else: mode = 'rb'
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -346,6 +346,14 @@
             with io.TextIOWrapper(f, encoding="ascii") as t:
                 self.assertEqual(t.readlines(), lines)
 
+    def test_fileobj_from_fdopen(self):
+        # Issue #13781: Opening a GzipFile for writing fails when using a
+        # fileobj created with os.fdopen().
+        fd = os.open(self.filename, os.O_WRONLY | os.O_CREAT)
+        with os.fdopen(fd, "wb") as f:
+            with gzip.GzipFile(fileobj=f, mode="w") as g:
+                pass
+
     # Testing compress/decompress shortcut functions
 
     def test_compress(self):