diff --git a/scrapy/utils/conf.py b/scrapy/utils/conf.py
index e8af90f11..435e9a6b3 100644
--- a/scrapy/utils/conf.py
+++ b/scrapy/utils/conf.py
@@ -1,5 +1,6 @@
 import os
 import sys
+import numbers
 from operator import itemgetter
 
 import six
@@ -34,6 +35,13 @@ def build_component_list(compdict, custom=None, convert=update_classpath):
             _check_components(compdict)
             return {convert(k): v for k, v in six.iteritems(compdict)}
 
+    def _validate_values(compdict):
+        """Fail if a value in the components dict is not a real number or None."""
+        for name, value in six.iteritems(compdict):
+            if value is not None and not isinstance(value, numbers.Real):
+                raise ValueError('Invalid value {} for component {}, please provide ' \
+                                 'a real number or None instead'.format(value, name))
+
     # BEGIN Backwards compatibility for old (base, custom) call signature
     if isinstance(custom, (list, tuple)):
         _check_components(custom)
@@ -43,6 +51,7 @@ def build_component_list(compdict, custom=None, convert=update_classpath):
         compdict.update(custom)
     # END Backwards compatibility
 
+    _validate_values(compdict)
     compdict = without_none_values(_map_keys(compdict))
     return [k for k, v in sorted(six.iteritems(compdict), key=itemgetter(1))]
 
diff --git a/tests/test_utils_conf.py b/tests/test_utils_conf.py
index dab41ac8d..f203c32ef 100644
--- a/tests/test_utils_conf.py
+++ b/tests/test_utils_conf.py
@@ -62,6 +62,27 @@ class BuildComponentListTest(unittest.TestCase):
         self.assertRaises(ValueError, build_component_list, duplicate_bs,
                           convert=lambda x: x.lower())
 
+    def test_valid_numbers(self):
+        # work well with None and numeric values
+        d = {'a': 10, 'b': None, 'c': 15, 'd': 5.0}
+        self.assertEqual(build_component_list(d, convert=lambda x: x),
+                         ['d', 'a', 'c'])
+        d = {'a': 33333333333333333333, 'b': 11111111111111111111, 'c': 22222222222222222222}
+        self.assertEqual(build_component_list(d, convert=lambda x: x),
+                         ['b', 'c', 'a'])
+        # raise exception for invalid values
+        d = {'one': '5'}
+        self.assertRaises(ValueError, build_component_list, {}, d, convert=lambda x: x)
+        d = {'one': '1.0'}
+        self.assertRaises(ValueError, build_component_list, {}, d, convert=lambda x: x)
+        d = {'one': [1, 2, 3]}
+        self.assertRaises(ValueError, build_component_list, {}, d, convert=lambda x: x)
+        d = {'one': {'a': 'a', 'b': 2}}
+        self.assertRaises(ValueError, build_component_list, {}, d, convert=lambda x: x)
+        d = {'one': 'lorem ipsum',}
+        self.assertRaises(ValueError, build_component_list, {}, d, convert=lambda x: x)
+
+
 
 class UtilsConfTestCase(unittest.TestCase):