diff --git a/src/etchdb/db.py b/src/etchdb/db.py
index 43fd16b..02a957a 100644
--- a/src/etchdb/db.py
+++ b/src/etchdb/db.py
@@ -255,9 +255,11 @@ async def iter_rows_keyset(
 
         - It must be a single column (composite-PK keyset uses
           `(a, b) > (last_a, last_b)` and isn't supported here).
-        - It must be monotonic-ordered and unique enough that no two
-          rows tie. Primary keys usually qualify; created_at columns
-          can if the resolution is high enough.
+        - It must be NOT NULL and unique enough that no two rows
+          tie. Primary keys usually qualify; created_at columns can
+          if the resolution is high enough. A NULL at a page
+          boundary stalls the cursor (WHERE by > NULL is false),
+          so we raise rather than loop forever.
 
         Defaults to `model.__pk__[0]`. Filters are AND'd with the
         cursor. Ordering is ascending; descending is not supported.
@@ -287,6 +289,12 @@ async def iter_rows_keyset(
             rows = await self._adapter.fetch(q.sql, *q.params)
             if not rows:
                 return
+            if len(rows) == batch_size and rows[-1][by] is None:
+                raise ValueError(
+                    f"iter_rows_keyset: {by!r} is NULL at the page boundary; "
+                    f"the cursor cannot advance. Pick a non-nullable column "
+                    f"for `by`, or filter NULLs out via raw SQL."
+                )
             for row in rows:
                 yield model(**row)
             if len(rows) < batch_size:
diff --git a/tests/integration/test_iter_rows_keyset.py b/tests/integration/test_iter_rows_keyset.py
index 67f9f7f..6901430 100644
--- a/tests/integration/test_iter_rows_keyset.py
+++ b/tests/integration/test_iter_rows_keyset.py
@@ -91,3 +91,20 @@ async def test_iter_rows_keyset_rejects_non_db_column(db: DB):
     with pytest.raises(ValueError, match="not a DB column"):
         async for _ in db.iter_rows_keyset(User, by="nonexistent_col"):
             pass
+
+
+async def test_iter_rows_keyset_raises_on_null_page_boundary(db: DB):
+    """NULL at a full-page boundary stalls the cursor (WHERE by >
+    NULL is false), so the loop would re-fetch the same page forever.
+    Raise instead. Three NULL emails + one non-NULL + batch_size=2
+    reproduces on both SQLite (NULLs first) and Postgres (NULLs
+    last), since either way a NULL row lands at the end of a full
+    non-final page."""
+    await db.insert(User(id=1, name="u1", email=None))
+    await db.insert(User(id=2, name="u2", email=None))
+    await db.insert(User(id=3, name="u3", email=None))
+    await db.insert(User(id=4, name="u4", email="a@x"))
+
+    with pytest.raises(ValueError, match="NULL at the page boundary"):
+        async for _ in db.iter_rows_keyset(User, by="email", batch_size=2):
+            pass