diff --git a/scispacy/hyponym_detector.py b/scispacy/hyponym_detector.py
index 02dd2505..2d0d5e84 100644
--- a/scispacy/hyponym_detector.py
+++ b/scispacy/hyponym_detector.py
@@ -62,6 +62,8 @@ def expand_to_noun_compound(self, token: Token, doc: Doc):
 
         start = token.i
         while True:
+            if start==0:
+                break
             previous = doc[start - 1]
             if previous.pos_ in {"PROPN", "NOUN", "PRON"}:
                 start -= 1
@@ -70,7 +72,10 @@ def expand_to_noun_compound(self, token: Token, doc: Doc):
 
         end = token.i + 1
         while True:
-            previous = doc[end]
+            try:
+                previous = doc[end]
+            except IndexError:
+                break
             if previous.pos_ in {"PROPN", "NOUN", "PRON"}:
                 end += 1
             else: