diff --git a/src/genomicranges/GenomicRanges.py b/src/genomicranges/GenomicRanges.py
index 9247436..337bf60 100644
--- a/src/genomicranges/GenomicRanges.py
+++ b/src/genomicranges/GenomicRanges.py
@@ -222,7 +222,7 @@ def _sanitize_seqnames(self, seqnames, seqinfo):
         if not isinstance(seqnames, np.ndarray):
             seqnames = np.asarray([self._reverse_seqindex[x] for x in seqnames])
 
-            num_uniq = len(np.unique(seqnames))
+            num_uniq = np.max(seqnames)
             if num_uniq < 2**8:
                 seqnames = seqnames.astype(np.int8)
             elif num_uniq < 2**16: