diff --git a/django/template/defaultfilters.py b/django/template/defaultfilters.py
index bb883d0fc6..e6a0c619dc 100644
--- a/django/template/defaultfilters.py
+++ b/django/template/defaultfilters.py
@@ -578,7 +578,7 @@ def length(value):
     try:
         return len(value)
     except (ValueError, TypeError):
-        return ''
+        return 0
 
 
 @register.filter(is_safe=False)
diff --git a/docs/ref/templates/builtins.txt b/docs/ref/templates/builtins.txt
index 9d4093e4da..e66c583720 100644
--- a/docs/ref/templates/builtins.txt
+++ b/docs/ref/templates/builtins.txt
@@ -1688,6 +1688,11 @@ For example::
 If ``value`` is ``['a', 'b', 'c', 'd']`` or ``"abcd"``, the output will be
 ``4``.
 
+.. versionchanged:: 1.8
+
+    The filter returns ``0`` for an undefined variable. Previously, it returned
+    an empty string.
+
 .. templatefilter:: length_is
 
 length_is
diff --git a/docs/releases/1.8.txt b/docs/releases/1.8.txt
index 31f4fb2cbb..1345e55a9a 100644
--- a/docs/releases/1.8.txt
+++ b/docs/releases/1.8.txt
@@ -267,6 +267,9 @@ Miscellaneous
 
 * ``django.contrib.gis`` dropped support for GEOS 3.1 and GDAL 1.6.
 
+* The :tfilter:`length` template filter now returns ``0`` for an undefined
+  variable, rather than an empty string.
+
 .. _deprecated-features-1.8:
 
 Features deprecated in 1.8
diff --git a/tests/template_tests/filters.py b/tests/template_tests/filters.py
index e13db66e96..ad7f79d888 100644
--- a/tests/template_tests/filters.py
+++ b/tests/template_tests/filters.py
@@ -322,8 +322,8 @@ def get_filter_tests():
         'length04': ('{{ string|length }}', {'string': 'django'}, '6'),
         'length05': ('{% if string|length == 6 %}Pass{% endif %}', {'string': mark_safe('django')}, 'Pass'),
         # Invalid uses that should fail silently.
-        'length06': ('{{ int|length }}', {'int': 7}, ''),
-        'length07': ('{{ None|length }}', {'None': None}, ''),
+        'length06': ('{{ int|length }}', {'int': 7}, '0'),
+        'length07': ('{{ None|length }}', {'None': None}, '0'),
 
         # length_is filter.
         'length_is01': ('{% if some_list|length_is:"4" %}Four{% endif %}', {'some_list': ['4', None, True, {}]}, 'Four'),