diff --git a/src/gradescopeapi/classes/_helpers/_course_helpers.py b/src/gradescopeapi/classes/_helpers/_course_helpers.py
index 23ab416..f2e6a7b 100644
--- a/src/gradescopeapi/classes/_helpers/_course_helpers.py
+++ b/src/gradescopeapi/classes/_helpers/_course_helpers.py
@@ -155,7 +155,9 @@ def get_course_members(soup: BeautifulSoup, course_id: str) -> list[Member]:
         # fetch other attributes: email, role, and section
         # from data attributes in button
         email = data_button.get("data-email")
-        role = id_to_role[data_button.get("data-role")]
+        # an unrecognised role id passes through as-is rather than sinking the whole roster
+        role_id = data_button.get("data-role")
+        role = id_to_role.get(role_id, role_id)
         sections = data_button.get("data-sections")  # TODO: check if this is correct
 
         # fetch user_id, only available on 'student' accounts
@@ -164,11 +166,16 @@ def get_course_members(soup: BeautifulSoup, course_id: str) -> list[Member]:
         user_id = None
         if rosterName_button is not None:
             # data-url="/courses/753413/gradebook.json?user_id=6515875"
-            data_url: str = rosterName_button.get("data-url", None)
-            user_id = data_url.split("user_id=")[-1]
-
-        # fetch number of submissions from table cell
-        num_submissions = int(cells[num_submissions_column].text)
+            data_url: str | None = rosterName_button.get("data-url", None)
+            if data_url is not None and "user_id=" in data_url:
+                user_id = data_url.split("user_id=")[-1]
+
+        # fetch number of submissions from table cell;
+        # the cell is blank for members who cannot submit (e.g. instructors, TAs, readers)
+        num_submissions_text = cells[num_submissions_column].text.strip()
+        num_submissions = (
+            int(num_submissions_text) if num_submissions_text.isdigit() else 0
+        )
 
         # create Member object with all relevant info
         member_list.append(
diff --git a/src/gradescopeapi/classes/account.py b/src/gradescopeapi/classes/account.py
index 93467a4..c260647 100644
--- a/src/gradescopeapi/classes/account.py
+++ b/src/gradescopeapi/classes/account.py
@@ -88,17 +88,12 @@ def get_course_users(self, course_id: str) -> list[Member]:
 
         session = self.session
 
-        try:
-            # scrape page
-            membership_resp = check_page_auth(session, membership_endpoint)
-            membership_soup = BeautifulSoup(membership_resp.text, "html.parser")
-
-            # get all users in the course
-            users = get_course_members(membership_soup, course_id)
+        # scrape page
+        membership_resp = check_page_auth(session, membership_endpoint)
+        membership_soup = BeautifulSoup(membership_resp.text, "html.parser")
 
-            return users
-        except Exception:
-            return None
+        # get all users in the course
+        return get_course_members(membership_soup, course_id)
 
     def get_assignments(self, course_id: str) -> list[Assignment]:
         """
diff --git a/tests/test_course_members.py b/tests/test_course_members.py
new file mode 100644
index 0000000..d9b704d
--- /dev/null
+++ b/tests/test_course_members.py
@@ -0,0 +1,97 @@
+"""Tests for roster parsing in get_course_members.
+
+These are offline tests against fixture HTML, so they need no Gradescope credentials.
+"""
+
+from bs4 import BeautifulSoup
+
+from gradescopeapi.classes._helpers._course_helpers import get_course_members
+
+COURSE_ID = "753413"
+
+
+def _roster(rows: str) -> BeautifulSoup:
+    html = f"""
+    <table class="js-rosterTable">
+      <tr><th>Name</th><th>Email</th><th>Role</th><th>Sections</th>
+          <th>Submissions</th><th>Edit</th><th>Remove</th></tr>
+      {rows}
+    </table>"""
+    return BeautifulSoup(html, "html.parser")
+
+
+def _row(data_cm: str, role: str, name_button: str, submissions: str) -> str:
+    return f"""
+    <tr class="rosterRow">
+      <td>
+        <button class="rosterCell--editIcon" data-cm='{data_cm}'
+                data-email="member@example.edu" data-role="{role}"
+                data-sections="A01"></button>
+        {name_button}
+      </td>
+      <td>e</td><td>r</td><td>s</td><td>{submissions}</td><td>edit</td><td>remove</td>
+    </tr>"""
+
+
+STUDENT_CM = '{"full_name":"Test Student","first_name":"Test","last_name":"Student","sid":"912345678"}'
+STAFF_CM = '{"full_name":"Test Instructor","first_name":"Test","last_name":"Instructor","sid":null}'
+
+
+def test_student_row_parses_user_id_and_submissions():
+    """A student row has a gradebook data-url, which is where user_id comes from."""
+    name_button = (
+        '<button class="js-rosterName" '
+        f'data-url="/courses/{COURSE_ID}/gradebook.json?user_id=6515875"></button>'
+    )
+    (member,) = get_course_members(
+        _roster(_row(STUDENT_CM, "0", name_button, "7")), COURSE_ID
+    )
+
+    assert member.full_name == "Test Student"
+    assert member.role == "Student"
+    assert member.sid == "912345678"
+    assert member.user_id == "6515875"
+    assert member.num_submissions == 7
+
+
+def test_staff_row_without_name_button_has_no_user_id():
+    """Only student accounts expose a user_id, and staff submission cells are blank."""
+    (member,) = get_course_members(_roster(_row(STAFF_CM, "1", "", "")), COURSE_ID)
+
+    assert member.full_name == "Test Instructor"
+    assert member.role == "Instructor"
+    assert member.user_id is None
+    assert member.num_submissions == 0
+
+
+def test_name_button_without_data_url_has_no_user_id():
+    """A js-rosterName button need not carry a data-url; that must not raise."""
+    name_button = '<button class="js-rosterName" data-name="Test Instructor"></button>'
+    (member,) = get_course_members(
+        _roster(_row(STAFF_CM, "2", name_button, "&mdash;")), COURSE_ID
+    )
+
+    assert member.role == "TA"
+    assert member.user_id is None
+    assert member.num_submissions == 0
+
+
+def test_unrecognised_role_id_passes_through():
+    """A role id outside the known set must not sink the roster."""
+    (member,) = get_course_members(_roster(_row(STAFF_CM, "9", "", "")), COURSE_ID)
+
+    assert member.role == "9"
+    assert member.full_name == "Test Instructor"
+
+
+def test_mixed_roster_returns_every_member():
+    """One unparseable row used to sink the whole roster."""
+    student_button = (
+        '<button class="js-rosterName" '
+        f'data-url="/courses/{COURSE_ID}/gradebook.json?user_id=6515875"></button>'
+    )
+    rows = _row(STUDENT_CM, "0", student_button, "7") + _row(STAFF_CM, "1", "", "")
+    members = get_course_members(_roster(rows), COURSE_ID)
+
+    assert [m.role for m in members] == ["Student", "Instructor"]
+    assert [m.user_id for m in members] == ["6515875", None]