diff --git a/gilda/ner.py b/gilda/ner.py
index bc0a25b..d904734 100644
--- a/gilda/ner.py
+++ b/gilda/ner.py
@@ -47,10 +47,8 @@
 
 from typing import List
 
-import nltk
 from nltk.corpus import stopwords
-from nltk.tokenize import sent_tokenize
-from nltk.tokenize import TreebankWordTokenizer
+from nltk.tokenize import PunktSentenceTokenizer, TreebankWordTokenizer
 
 from gilda import get_grounder
 from gilda.grounder import Annotation
@@ -105,14 +103,20 @@ def annotate(
     """
     if grounder is None:
         grounder = get_grounder()
+    sent_tokenizer = PunktSentenceTokenizer()
     if sent_split_fun is None:
-        sent_split_fun = sent_tokenize
+        sent_split_fun = sent_tokenizer.tokenize
     # Get sentences
     sentences = sent_split_fun(text)
+    sentence_coords = list(sent_tokenizer.span_tokenize(text))
     text_coord = 0
     annotations = []
     word_tokenizer = TreebankWordTokenizer()
-    for sentence in sentences:
+    # FIXME: a custom sentence split function can be inconsistent
+    # with the coordinates being used here which come from NLTK
+    for sentence, sentence_coord in zip(sentences, sentence_coords):
+        # FIXME: one rare corner case is named entities with single quotes
+        # in them which get tokenized in a weird way
         raw_word_coords = \
             list(word_tokenizer.span_tokenize(sentence.rstrip('.')))
         raw_words = [sentence[start:end] for start, end in raw_word_coords]
@@ -143,7 +147,8 @@ def annotate(
                                     raw_word_coords[idx:idx+span]):
                     # Figure out if we need a space before this word, then
                     # append the word.
-                    spaces = ' ' * (c[0] - len(raw_span) - raw_word_coords[idx][0])
+                    spaces = ' ' * (c[0] - len(raw_span) -
+                                    raw_word_coords[idx][0])
                     txt_span += spaces + w
                     raw_span += spaces + rw
                 context = text if context_text is None else context_text
@@ -152,8 +157,9 @@ def annotate(
                                           organisms=organisms,
                                           namespaces=namespaces)
                 if matches:
-                    start_coord = raw_word_coords[idx][0]
-                    end_coord = raw_word_coords[idx+span-1][1]
+                    start_coord = sentence_coord[0] + raw_word_coords[idx][0]
+                    end_coord = sentence_coord[0] + \
+                        raw_word_coords[idx+span-1][1]
                     annotations.append(Annotation(
                         raw_span, matches, start_coord, end_coord
                     ))
diff --git a/gilda/tests/test_ner.py b/gilda/tests/test_ner.py
index 6c54758..c8b24c1 100644
--- a/gilda/tests/test_ner.py
+++ b/gilda/tests/test_ner.py
@@ -83,9 +83,9 @@ def test_context_test():
     context_text = "Calcium is released from the ER."
     results = gilda.annotate(text, context_text=context_text)
     assert len(results) == 1
-    assert results[0].matches[0].term.get_curie() == "GO:0005783"
-    assert results[0].text == "ER"
-    assert (results[0].start, results[0].end) == (14, 16)
+    assert results[1].matches[0].term.get_curie() == "GO:0005783"
+    assert results[1].text == "ER"
+    assert (results[1].start, results[0].end) == (14, 16)
 
 
 def test_punctuation_comma_in_entity():