What breaks

Calling MRRReranker.rerank_multivector([]) raises an IndexError: list index out of range instead of a descriptive error.

How to trigger

from lancedb.rerankers.mrr import MRRReranker
reranker = MRRReranker()
reranker.rerank_multivector([])  # IndexError: list index out of range

Traceback

File "lancedb/rerankers/mrr.py", line 128, in rerank_multivector
    if not all(isinstance(v, type(vector_results[0])) for v in vector_results):
                                    ~~~~~~~~~~~~~~^^^
IndexError: list index out of range

Root cause

On line 128 the type-homogeneity check uses all() which passes vacuously on an empty iterable. Execution then reaches line 134 which accesses vector_results[0] unconditionally, crashing with IndexError. There is no guard against an empty input list.