diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -165,6 +165,7 @@
         # normalization, ensuring compatibility with our ui type.
 
         # write_through is new in Python 3.7.
+        sys.stdout = sys.stdout or io.TextIOWrapper(io.BytesIO())
         kwargs = {
             "newline": "\n",
             "line_buffering": sys.stdout.line_buffering,
@@ -175,6 +176,7 @@
             sys.stdout.buffer, sys.stdout.encoding, sys.stdout.errors, **kwargs
         )
 
+        sys.stderr = sys.stderr or io.TextIOWrapper(io.BytesIO())
         kwargs = {
             "newline": "\n",
             "line_buffering": sys.stderr.line_buffering,
@@ -186,6 +188,7 @@
         )
 
         # No write_through on read-only stream.
+        sys.stdin = sys.stdin or io.TextIOWrapper(io.BytesIO())
         sys.stdin = io.TextIOWrapper(
             sys.stdin.buffer,
             sys.stdin.encoding,
diff --git a/mercurial/utils/procutil.py b/mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py
+++ b/mercurial/utils/procutil.py
@@ -114,12 +114,17 @@
 
 
 if pycompat.ispy3:
+    # Stdio objects can be 'None' on Python 3. Most code paths (for example,
+    # dispatch.initstdio) do not expect that.
+    stdin = sys.stdin or io.TextIOWrapper(io.BytesIO())
+    stdout = sys.stdout or io.TextIOWrapper(io.BytesIO())
+    stderr = sys.stderr or io.TextIOWrapper(io.BytesIO())
     # Python 3 implements its own I/O streams.
     # TODO: .buffer might not exist if std streams were replaced; we'll need
     # a silly wrapper to make a bytes stream backed by a unicode one.
-    stdin = sys.stdin.buffer
-    stdout = _make_write_all(sys.stdout.buffer)
-    stderr = _make_write_all(sys.stderr.buffer)
+    stdin = stdin.buffer
+    stdout = _make_write_all(stdout.buffer)
+    stderr = _make_write_all(stderr.buffer)
     if pycompat.iswindows:
         # Work around Windows bugs.
         stdout = platform.winstdout(stdout)
diff --git a/tests/test-stdio-missing.t b/tests/test-stdio-missing.t
new file mode 100644
--- /dev/null
+++ b/tests/test-stdio-missing.t
@@ -0,0 +1,13 @@
+  $ cat > prompt.py << 'EOF'
+  > from mercurial import exthelper
+  > eh = exthelper.exthelper()
+  > cmdtable = eh.cmdtable
+  > @eh.command(b'prompt', [], norepo=True)
+  > def prompt(ui):
+  >     chosen = ui.promptchoice(b"is stdin present? (y/N) $$ &Yes $$ &No", default=1)
+  >     ui.write(b"chosen: %d\n" % chosen)
+  > EOF
+
+  $ hg --config extensions.prompt=prompt.py prompt 0<&-
+  is stdin present? (y/N)  n
+  chosen: 1