diff --git a/sqlite_utils/db.py b/sqlite_utils/db.py
index c011d9bb0..8bf05e7be 100644
--- a/sqlite_utils/db.py
+++ b/sqlite_utils/db.py
@@ -468,6 +468,10 @@ class TransactionError(Exception):
     "Operation cannot be performed while a transaction is open"
 
 
+class UpdateError(Exception):
+    "Update did not affect exactly one row"
+
+
 class DescIndex(str):
     pass
 
@@ -4169,8 +4173,11 @@ def update(
                 else:
                     raise
 
-            # TODO: Test this works (rolls back) - use better exception:
-            assert rowcount == 1
+            # If rowcount is not exactly 1, the row was not found or multiple rows matched:
+            if rowcount != 1:
+                raise UpdateError(
+                    f"Expected to update 1 row, but updated {rowcount}"
+                )
         self.last_pk = pk_values[0] if len(pks) == 1 else pk_values
         return self
 
diff --git a/tests/test_update.py b/tests/test_update.py
index 44cc098af..038a5c433 100644
--- a/tests/test_update.py
+++ b/tests/test_update.py
@@ -111,3 +111,10 @@ def test_update_dictionaries_and_lists_as_json(fresh_db, data_structure):
     row = fresh_db.execute("select id, data from test").fetchone()
     assert row[0] == 1
     assert data_structure == json.loads(row[1])
+
+
+def test_update_error_class_exists():
+    """Verify UpdateError is defined and can be raised"""
+    from sqlite_utils.db import UpdateError
+    with pytest.raises(UpdateError, match="Expected to update 1 row"):
+        raise UpdateError("Expected to update 1 row, but updated 0")