diff --git a/src/calibre/ebooks/html_entities.h b/src/calibre/ebooks/html_entities.h
index 03f6d17520..073326ab4c 100644
--- a/src/calibre/ebooks/html_entities.h
+++ b/src/calibre/ebooks/html_entities.h
@@ -1,5 +1,5 @@
 /* ANSI-C code produced by gperf version 3.1 */
-/* Command-line: gperf -t --readonly --includes  */
+/* Command-line: gperf --struct-type --readonly --includes  */
 /* Computed positions: -k'1-7,10,12,$' */
 
 #if !((' ' == 32) && ('!' == 33) && ('"' == 34) && ('#' == 35) \
diff --git a/src/calibre/ebooks/html_entities.py b/src/calibre/ebooks/html_entities.py
index 0456d9ceaa..b1a9d969cb 100644
--- a/src/calibre/ebooks/html_entities.py
+++ b/src/calibre/ebooks/html_entities.py
@@ -2135,13 +2135,26 @@ html5_entities = {
 }
 
 
+def find_tests():
+    import unittest
+    class TestHTMLEntityReplacement(unittest.TestCase):
+        def test_html_entity_replacement(self):
+            from calibre_extensions.fast_html_entities import replace_entities
+            def t(inp, exp):
+                self.assertEqual(exp, replace_entities(inp), f'Failed for input: {inp!r}')
+            t('', '')
+            t('a', 'a')
+
+    return unittest.defaultTestLoader.loadTestsFromTestCase(TestHTMLEntityReplacement)
+
+
 def generate_entity_lists():
     import re
     from html import entities as e
     entities = {k.rstrip(';'): e.name2codepoint[k] for k in e.name2codepoint}
     entities.update({k.rstrip(';'): e.html5[k] for k in e.html5})
     # common misspelled entity names
-    for k, v in {'apos': "'", 'squot': "'", 'hellips': entities['hellip']}.items():
+    for k, v in {'squot': "'", 'hellips': entities['hellip']}.items():
         if k not in entities:
             entities[k] = v
     lines = []
diff --git a/src/calibre/utils/run_tests.py b/src/calibre/utils/run_tests.py
index 71128c243e..699582961a 100644
--- a/src/calibre/utils/run_tests.py
+++ b/src/calibre/utils/run_tests.py
@@ -269,6 +269,8 @@ def find_tests(which_tests=None, exclude_tests=None):
         a(find_tests())
         from calibre.gui2.viewer.annotations import find_tests
         a(find_tests())
+        from calibre.ebooks.html_entities import find_tests
+        a(find_tests())
     if ok('misc'):
         from calibre.ebooks.html.input import find_tests
         a(find_tests())