diff --git a/src/qodev_apollo_api/client.py b/src/qodev_apollo_api/client.py
index e862cde..bb895c3 100644
--- a/src/qodev_apollo_api/client.py
+++ b/src/qodev_apollo_api/client.py
@@ -545,7 +545,26 @@ async def update_opportunity_roles(
         Returns:
             The updated Deal.
         """
-        data = {"opportunity_id": opportunity_id, "roles": roles}
+        # Apollo's endpoint expects each entry's role type *nested* under a ``role``
+        # array — sending ``opportunity_contact_role_type_id`` flat on the entry (with
+        # no ``role`` key) makes the server call ``.map`` on nil and 422 with
+        # "undefined method 'map' for nil". Reshape the flat RoleAssignment entries
+        # into the wire format the server actually accepts.
+        wire_roles: list[dict] = []
+        for entry in roles:
+            # RoleAssignment types is_primary as a bool; default to False when omitted.
+            # Avoid bool(...) coercion, which would turn a stray truthy non-bool (e.g.
+            # the string "false") into True.
+            is_primary = entry.get("is_primary", False)
+            role_obj: dict[str, Any] = {"is_primary": is_primary}
+            role_type_id = entry.get("opportunity_contact_role_type_id")
+            if role_type_id:
+                role_obj["opportunity_contact_role_type_id"] = role_type_id
+            wire_roles.append(
+                {"contact_id": entry["contact_id"], "is_primary": is_primary, "role": [role_obj]}
+            )
+
+        data = {"opportunity_id": opportunity_id, "roles": wire_roles}
         result = await self._post("/opportunities/update_roles", data)
         return Deal.model_validate(result.get("opportunity", result))
 
diff --git a/tests/test_client.py b/tests/test_client.py
index 6406a78..8ffefa3 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -643,7 +643,11 @@ async def test_list_opportunity_contact_role_types(client: ApolloClient):
 
 
 async def test_update_opportunity_roles(client: ApolloClient):
-    """Test POST /opportunities/update_roles returns the updated Deal with the roles body."""
+    """The flat RoleAssignment entries are reshaped into Apollo's nested ``role`` wire format.
+
+    Regression: sending ``opportunity_contact_role_type_id`` flat on the entry (no
+    ``role`` key) makes Apollo 422 with "undefined method 'map' for nil".
+    """
     client._client.request.return_value = _make_response(
         {"opportunity": {"id": "d1", "name": "Big Deal"}}
     )
@@ -659,7 +663,19 @@ async def test_update_opportunity_roles(client: ApolloClient):
 
     call_args = client._client.request.call_args
     assert call_args[0] == ("POST", "/opportunities/update_roles")
-    assert call_args[1]["json"] == {"opportunity_id": "d1", "roles": roles}
+    assert call_args[1]["json"] == {
+        "opportunity_id": "d1",
+        "roles": [
+            {
+                "contact_id": "c1",
+                "is_primary": True,
+                "role": [{"is_primary": True, "opportunity_contact_role_type_id": "rt1"}],
+            },
+            # No role type → the nested role object carries only is_primary (never a
+            # flat/absent role type, which is what triggered the nil.map crash).
+            {"contact_id": "c2", "is_primary": False, "role": [{"is_primary": False}]},
+        ],
+    }
 
 
 async def test_list_custom_fields(client: ApolloClient):