What

_merge_config silently ignored a top-level non-mapping bundle-catalogs.yml (a YAML list or scalar): data.get("catalogs") if isinstance(data, dict) else None made a list/scalar document fall through to None → early return → the built-in default stack, with no error.

The sibling reader of the same file — commands_impl/catalog_config._read — raises "expected a mapping at the top level" for exactly those documents. Merged PR #3623 already made the inner non-list catalogs value agree between the two readers; this closes the remaining top-level-shape gap so both readers reject the same malformed input.

Fix

Add an explicit top-level mapping guard mirroring _read:

if not isinstance(data, dict):
    raise BundlerError(f"Malformed catalog config at {config_path}: expected a mapping at the top level, got {type(data).__name__}.")

An empty file (load_yaml coerces to {}), absent catalogs, and catalogs: [] all remain no-ops.

Tests

tests/contract/test_catalog_schema.py::test_toplevel_non_mapping_raises (parametrized over a top-level list and a scalar) — both raise after the fix, silently return the default stack before it. Regression tests confirm absent/empty catalogs still yield the built-in defaults. ruff clean; all 23 contract tests pass.

AI-assisted: authored with Claude Code. Verified the message/behavior matches the _read sibling and confirmed fail-before/pass-after.