diff --git a/ehrql/query_engines/in_memory_database.py b/ehrql/query_engines/in_memory_database.py
index f2484fcb1..ce5e9feb9 100644
--- a/ehrql/query_engines/in_memory_database.py
+++ b/ehrql/query_engines/in_memory_database.py
@@ -208,7 +208,7 @@ def sort(self, sort_index):
     def pick_at_index(self, ix):
         return PatientTable(
             {
-                name: col.pick_at_index(ix, name == "patient_id")
+                name: col.pick_at_index(ix)
                 for name, col in self.name_to_col.items()
                 if name != "row_id"
             }
@@ -351,14 +351,19 @@ def sort(self, sort_index):
             {p: rows.sort(sort_index[p]) for p, rows in self.patient_to_rows.items()}
         )
 
-    def pick_at_index(self, ix, is_patient_id=False):
-        if is_patient_id:
-            # The patient_id column is special, and should always be a mapping
-            # from an id to itself.  Rows.pick_at_index will return None if a
-            # patient has no rows in the column.
-            return PatientColumn({p: p for p in self.patient_to_rows})
+    def pick_at_index(self, ix):
+        # It is arguable that for a patient with no rows (which would occur if
+        # this EventColumn was derived by filtering another EventColumn), the
+        # patient should be present in the new PatientColumn, with value None.
+        #
+        # However, we have decided to instead omit the patient from the new
+        # PatientColumn.
         return PatientColumn(
-            {p: rows.pick_at_index(ix) for p, rows in self.patient_to_rows.items()}
+            {
+                p: rows.pick_at_index(ix)
+                for p, rows in self.patient_to_rows.items()
+                if rows
+            }
         )
 
 
@@ -430,10 +435,7 @@ def sort(self, sort_index):
     def pick_at_index(self, ix):
         """Return element at given position."""
 
-        try:
-            k = list(self)[ix]
-        except IndexError:
-            return None
+        k = list(self)[ix]
         return self[k]
 
 
diff --git a/tests/integration/test_query_engines.py b/tests/integration/test_query_engines.py
index a04497d0a..b26338d9f 100644
--- a/tests/integration/test_query_engines.py
+++ b/tests/integration/test_query_engines.py
@@ -350,3 +350,23 @@ def test_population_which_uses_combine_as_set_and_no_patient_frame(engine):
     assert engine.extract_qm(variables) == [
         {"patient_id": 1, "v": True},
     ]
+
+
+def test_picking_row_doesnt_cause_filtered_rows_to_reappear(engine):
+    # Regression test for a bug we introduced in the in-memory engine
+    dataset = create_dataset()
+    dataset.define_population(events.exists_for_patient())
+
+    rows = events.where(events.i < 0).sort_by(events.i).first_for_patient()
+    dataset.has_row = rows.exists_for_patient()
+    dataset.row_count = rows.count_for_patient()
+
+    engine.populate(
+        {
+            events: [{"patient_id": 1, "i": 2}],
+        }
+    )
+
+    assert engine.extract(dataset) == [
+        {"patient_id": 1, "has_row": False, "row_count": 0},
+    ]
diff --git a/tests/unit/query_engines/test_in_memory_database.py b/tests/unit/query_engines/test_in_memory_database.py
index c3a592143..cf91646c7 100644
--- a/tests/unit/query_engines/test_in_memory_database.py
+++ b/tests/unit/query_engines/test_in_memory_database.py
@@ -395,7 +395,6 @@ def test_event_table_filter_then_pick_at_index():
         --+-----+-----
         1 | 102 | 112
         2 | 203 | 211
-        3 |     |
         """,
     )
 
diff --git a/tests/unit/test_quiz.py b/tests/unit/test_quiz.py
index 347b56560..48e2901bb 100644
--- a/tests/unit/test_quiz.py
+++ b/tests/unit/test_quiz.py
@@ -12,6 +12,7 @@
     clinical_events,
     medications,
     patients,
+    practice_registrations,
 )
 
 
@@ -159,6 +160,25 @@ def test_check_answer_dataset_column_has_missing_patients(engine):
     assert msg == "Incorrect `age` value for patient 1: expected 49, got 50 instead."
 
 
+@pytest.mark.parametrize(
+    "order, message",
+    [
+        ([0, 1], "Missing patient(s): 7."),
+        ([1, 0], "Found extra patient(s): 7."),
+    ],
+)
+def test_check_answer_patient_series_has_missing_or_extra_patients(
+    engine, order, message
+):
+    series = [
+        practice_registrations.for_patient_on("2013-12-01").practice_pseudo_id,
+        practice_registrations.for_patient_on("2014-01-01").practice_pseudo_id,
+    ]
+    answer, expected = (series[i] for i in order)
+    msg = quiz.check_answer(engine=engine, answer=answer, expected=expected)
+    assert msg == message
+
+
 def test_check_answer_patient_series_has_incorrect_value(engine):
     msg = quiz.check_answer(
         engine=engine,