fix: a card.json with null dates renders instead of crashing

emulo.py

@@ -2364,11 +2364,14 @@ def write_stats(result, out_dir):
     return stats
 
 def months_between(first_date, last_date):
+    # TypeError covers a reducer that emits JSON null (or any non-sliceable
+    # value) for a date: 0 means "unknown", and both card renderers already
+    # skip the months row when it is falsy, like every other missing stat.
     try:
         y1, m1 = int(first_date[:4]), int(first_date[5:7])
         y2, m2 = int(last_date[:4]), int(last_date[5:7])
         return max(1, (y2 - y1) * 12 + (m2 - m1) + 1)
-    except (ValueError, IndexError):
+    except (TypeError, ValueError, IndexError):
         return 0
 
 def fmt_tokens(n):

tests/test_card_null_dates.py

@@ -0,0 +1,119 @@
+"""A card.json whose stats carry JSON nulls must still render.
+
+A reducer that emits `"first_date": null` instead of `""` used to crash
+`emulo --card`: months_between() did `None[:4]`, which raises TypeError, and
+TypeError was not in the except tuple, so the traceback escaped both
+print_card() and render_card_html().
+
+Missing dates degrade the same way every other missing stat does on this card:
+months_between() returns 0 and both renderers already skip the months row when
+it is falsy.
+"""
+
+import contextlib
+import importlib.util
+import io
+import json
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+EMULO = ROOT / "emulo.py"
+SPEC = importlib.util.spec_from_file_location("emulo_card_nulls", EMULO)
+emulo = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(emulo)
+
+
+def card_with(stats):
+    return {
+        "archetype": "Proof-First Builder",
+        "laws": [{"text": "Always prove done.", "count": "12 sessions"}],
+        "truth": "you ship faster than you verify",
+        "stats": stats,
+    }
+
+
+class MonthsBetweenNullDatesTest(unittest.TestCase):
+    def test_null_dates_degrade_to_zero_instead_of_raising(self):
+        self.assertEqual(emulo.months_between(None, None), 0)
+        self.assertEqual(emulo.months_between(None, "2026-07-01"), 0)
+        self.assertEqual(emulo.months_between("2025-11-01", None), 0)
+
+    def test_non_string_dates_degrade_to_zero(self):
+        # a reducer emitting an epoch int or a list is just as unrenderable
+        self.assertEqual(emulo.months_between(20251101, 20260701), 0)
+        self.assertEqual(emulo.months_between(["2025-11-01"], ["2026-07-01"]), 0)
+        self.assertEqual(emulo.months_between({}, {}), 0)
+
+    def test_empty_and_malformed_strings_still_degrade_to_zero(self):
+        self.assertEqual(emulo.months_between("", ""), 0)
+        self.assertEqual(emulo.months_between("not-a-date", "also-not"), 0)
+
+    def test_real_dates_still_compute_the_span_inclusive(self):
+        self.assertEqual(emulo.months_between("2025-11-03", "2026-07-21"), 9)
+        self.assertEqual(emulo.months_between("2026-07-01", "2026-07-21"), 1)
+
+
+class CardRendersWithNullDatesTest(unittest.TestCase):
+    def test_render_card_html_survives_null_dates_and_omits_months(self):
+        html = emulo.render_card_html(card_with({
+            "sessions": 1656,
+            "tokens": 3_000_000,
+            "first_date": None,
+            "last_date": None,
+        }))
+        self.assertIn("1,656", html)
+        self.assertNotIn("<span>months</span>", html)
+
+    def test_render_card_html_keeps_months_when_dates_are_real(self):
+        html = emulo.render_card_html(card_with({
+            "sessions": 1656,
+            "first_date": "2025-11-03",
+            "last_date": "2026-07-21",
+        }))
+        self.assertIn("<span>months</span>", html)
+
+    def test_print_card_survives_null_dates(self):
+        buf = io.StringIO()
+        with contextlib.redirect_stdout(buf):
+            emulo.print_card(card_with({
+                "sessions": 1656,
+                "first_date": None,
+                "last_date": None,
+            }), still=True)
+        out = buf.getvalue()
+        self.assertIn("1,656 sessions", out)
+        self.assertNotIn("months", out)
+
+
+class CardCliWithNullDatesTest(unittest.TestCase):
+    def test_emulo_card_exits_clean_on_a_card_with_null_dates(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            out = Path(tmp) / "emulo-out"
+            out.mkdir()
+            card_path = out / "card.json"
+            card_path.write_text(json.dumps(card_with({
+                "sessions": 1656,
+                "messages": 42000,
+                "tokens": 3_000_000,
+                "first_date": None,
+                "last_date": None,
+            })), encoding="utf-8")
+            env = dict(os.environ, EMULO_NO_ANIM="1", PYTHONIOENCODING="utf-8")
+            result = subprocess.run(
+                [sys.executable, str(EMULO), "--card", str(card_path),
+                 "--out", str(out), "--no-open", "--still"],
+                capture_output=True, text=True, env=env,
+            )
+            self.assertEqual(result.returncode, 0, result.stderr)
+            self.assertNotIn("Traceback", result.stderr)
+            self.assertTrue((out / "card.html").exists())
+
+
+if __name__ == "__main__":
+    unittest.main()