diff --git a/Lib/gettext.py b/Lib/gettext.py
--- a/Lib/gettext.py
+++ b/Lib/gettext.py
@@ -81,15 +81,19 @@ def c2py(plural):
                                            expr.sub(repl, x.group(3)))
 
     stack = ['']
+    maybe_func = False
     for mo in re.finditer(_TOKEN_PATTERN, plural):
         kind = mo.lastgroup
         value = mo.group(kind)
         if kind == 'NUMBER' or kind == 'IDENTIFIER':
             stack[-1] += value
+            maybe_func = True
         elif kind == 'WHITESPACE':
             continue
         elif kind == 'PARENTHESIS':
             if value == '(':
+                if maybe_func:
+                    raise ValueError('function call in plural form')
                 stack.append('')
             else:
                 if len(stack) == 1:
@@ -105,6 +109,7 @@ def c2py(plural):
                 stack[-1] += ' not '
             else:
                 stack[-1] += value
+            maybe_func = False
         else:
             raise ValueError('invalid token in plural form: {}'.format(value))
 
diff --git a/Lib/test/test_gettext.py b/Lib/test/test_gettext.py
--- a/Lib/test/test_gettext.py
+++ b/Lib/test/test_gettext.py
@@ -365,6 +365,10 @@ class PluralFormsTestCase(GettextBaseTest):
         # Test for a dangerous expression
         raises(ValueError, gettext.c2py, "os.chmod('/etc/passwd',0777)")
 
+    def test_security_function_call(self):
+        raises = self.assertRaises
+        raises(ValueError, gettext.c2py, "n()")
+
 class GNUTranslationParsingTest(GettextBaseTest):
     def test_plural_form_error_issue17898(self):
         with open(MOFILE, 'wb') as fp: