diff --git a/openvpncertupdate.py b/openvpncertupdate.py
index cc6ecaa..592f801 100644
--- a/openvpncertupdate.py
+++ b/openvpncertupdate.py
@@ -651,9 +651,15 @@ def show_confirm(stdscr, message: str) -> bool:
     """Y/N modal dialog. Returns True if user presses y/Y."""
     sh, sw = stdscr.getmaxyx()
     lines  = message.splitlines()
-    h = len(lines) + 6
-    w = max(max(len(l) for l in lines) + 6, 38)
-    win = curses.newwin(h, w, (sh - h) // 2, (sw - w) // 2)
+    # Clamp to the screen: an unbounded width (e.g. a long Cryptgeon URL line)
+    # makes curses.newwin() raise "curses function returned NULL" outright on
+    # a narrow terminal (e.g. a serial console) instead of just wrapping badly.
+    h = min(len(lines) + 6, sh)
+    w = min(max(max(len(l) for l in lines) + 6, 38), sw)
+    try:
+        win = curses.newwin(h, w, max(0, (sh - h) // 2), max(0, (sw - w) // 2))
+    except curses.error:
+        return False
     win.keypad(True)
     draw_box(win, "Confirm")
     for i, line in enumerate(lines):
@@ -704,8 +710,14 @@ def show_cert_form(
     Tab / Shift-Tab / Up / Down cycle fields and buttons. F5 regenerates password.
     Enter or Space activates the focused button. Ctrl-G confirms immediately. Escape cancels."""
     sh, sw = stdscr.getmaxyx()
-    h, w   = 13, 62
-    win    = curses.newwin(h, w, (sh - h) // 2, (sw - w) // 2)
+    # Clamp to the screen so curses.newwin() can't raise "curses function
+    # returned NULL" outright on a terminal narrower/shorter than the usual
+    # 80x24 (e.g. a serial console with an unusually small viewport).
+    h, w   = min(13, sh), min(62, sw)
+    try:
+        win = curses.newwin(h, w, max(0, (sh - h) // 2), max(0, (sw - w) // 2))
+    except curses.error:
+        return CertFormResult(cn=cn, email="", password="", confirmed=False)
     win.keypad(True)
     fw = w - 6   # field width
 
diff --git a/tests/test_dialogs.py b/tests/test_dialogs.py
index 2b9a8df..ad18bb2 100644
--- a/tests/test_dialogs.py
+++ b/tests/test_dialogs.py
@@ -62,6 +62,31 @@ def test_show_confirm_esc():
     assert result is False
 
 
+def test_show_confirm_clamps_width_to_screen_for_long_line():
+    """A long message line (e.g. a Cryptgeon password URL) must not make the
+    requested newwin() width exceed the screen — that's what made
+    curses.newwin() raise 'curses function returned NULL' on a narrow
+    terminal such as a serial console."""
+    stdscr = _make_stdscr(rows=24, cols=80)
+    win = _make_win()
+    win.getch.side_effect = [ord("y")]
+    long_url = "https://cryptgeon.example.com/note/" + "a" * 100 + "#" + "b" * 64
+    with patch("curses.newwin", return_value=win) as mock_newwin:
+        show_confirm(stdscr, f"Send email?\n\nPassword URL: {long_url}")
+    h, w, y, x = mock_newwin.call_args.args
+    assert w <= 80
+    assert x >= 0
+
+
+def test_show_confirm_newwin_failure_returns_false():
+    """If curses.newwin() still fails despite clamping, show_confirm must
+    degrade to 'declined' rather than crash the whole app."""
+    stdscr = _make_stdscr()
+    with patch("curses.newwin", side_effect=_curses.error("returned NULL")):
+        result = show_confirm(stdscr, "Do you want to continue?")
+    assert result is False
+
+
 # ---------------------------------------------------------------------------
 # show_cert_form tests
 # ---------------------------------------------------------------------------
@@ -102,3 +127,28 @@ def test_show_cert_form_cancel_button():
         result = show_cert_form(stdscr, cn="alice", email="alice@example.com")
     assert isinstance(result, CertFormResult)
     assert result.confirmed is False
+
+
+def test_show_cert_form_clamps_to_narrow_screen():
+    """The form's fixed 13x62 size must not exceed a smaller-than-usual
+    screen, which would make curses.newwin() raise."""
+    stdscr = _make_stdscr(rows=10, cols=40)
+    win = _make_win()
+    win.getch.side_effect = [27]
+    with patch("curses.newwin", return_value=win) as mock_newwin:
+        show_cert_form(stdscr, cn="alice", email="alice@example.com")
+    h, w, y, x = mock_newwin.call_args.args
+    assert h <= 10
+    assert w <= 40
+    assert y >= 0
+    assert x >= 0
+
+
+def test_show_cert_form_newwin_failure_returns_cancelled():
+    """If curses.newwin() still fails despite clamping, show_cert_form must
+    degrade to an unconfirmed result rather than crash the whole app."""
+    stdscr = _make_stdscr()
+    with patch("curses.newwin", side_effect=_curses.error("returned NULL")):
+        result = show_cert_form(stdscr, cn="alice", email="alice@example.com")
+    assert isinstance(result, CertFormResult)
+    assert result.confirmed is False