diff --git a/spacy/ml/models/coref_util.py b/spacy/ml/models/coref_util.py
index b0a632bd8..74bfbf6f0 100644
--- a/spacy/ml/models/coref_util.py
+++ b/spacy/ml/models/coref_util.py
@@ -128,7 +128,8 @@ def get_candidate_mentions(
         si = sentence_map[tok.i]  # sentence index
         for ii in range(1, max_span_width):
             ei = tok.i + ii  # end index
-            if ei >= len(doc) or sentence_map[ei] != si:
+            # Note: this matches slice syntax, so the token index is one less
+            if ei > len(doc) or sentence_map[ei-1] != si:
                 continue
 
             begins.append(tok.i)