--- a/Lib/gettext.py
+++ b/Lib/gettext.py
@@ -387,8 +387,9 @@ class GNUTranslations(NullTranslations):
                         self._charset = v.split('charset=')[1]
                     elif k == 'plural-forms':
                         v = v.split(';')
-                        plural = v[1].split('plural=')[1]
-                        self.plural = c2py(plural)
+                        if len(v) > 1:
+                            plural = v[1].split('plural=')[1]
+                            self.plural = c2py(plural)
             # Note: we unconditionally convert both msgids and msgstrs to
             # Unicode using the character encoding specified in the charset
             # parameter of the Content-Type header.  The gettext documentation