diff --git a/plugins/Math/plugin.py b/plugins/Math/plugin.py
index c34891461..01c9261fc 100644
--- a/plugins/Math/plugin.py
+++ b/plugins/Math/plugin.py
@@ -103,7 +103,8 @@ class Math(callbacks.Plugin):
     _mathEnv['abs'] = abs
     _mathEnv['max'] = max
     _mathEnv['min'] = min
-    _mathEnv.pop('factorial')
+    _mathSafeEnv = dict([(x,y) for x,y in _mathEnv.items()
+        if x not in ['factorial']])
     _mathRe = re.compile(r'((?:(?<![A-Fa-f\d)])-)?'
                          r'(?:0x[A-Fa-f\d]+|'
                          r'0[0-7]+|'
@@ -192,7 +193,7 @@ class Math(callbacks.Plugin):
         text = self._mathRe.sub(handleMatch, text)
         try:
             self.log.info('evaluating %q from %s', text, msg.prefix)
-            x = complex(eval(text, self._mathEnv, self._mathEnv))
+            x = complex(eval(text, self._mathSafeEnv, self._mathSafeEnv))
             irc.reply(self._complexToString(x))
         except OverflowError:
             maxFloat = math.ldexp(0.9999999999999999, 1024)
diff --git a/plugins/Math/test.py b/plugins/Math/test.py
index 36215326e..7dfb921e6 100644
--- a/plugins/Math/test.py
+++ b/plugins/Math/test.py
@@ -110,6 +110,7 @@ class MathTestCase(PluginTestCase):
         self.assertNotError('calc (1600 * 1200) - 2*(1024*1280)')
         self.assertNotError('calc 3-2*4')
         self.assertNotError('calc (1600 * 1200)-2*(1024*1280)')
+        self.assertError('calc factorial(99)')
 
     def testCalcNoNameError(self):
         self.assertNotRegexp('calc foobar(x)', 'NameError')
@@ -134,6 +135,7 @@ class MathTestCase(PluginTestCase):
         self.assertResponse('icalc 1^1', '0')
         self.assertResponse('icalc 10**24', '1' + '0'*24)
         self.assertRegexp('icalc 49/6', '8.16')
+        self.assertNotError('icalc factorial(99)')
 
     def testRpn(self):
         self.assertResponse('rpn 5 2 +', '7')