diff --git a/app/helpers/avatars_helper.rb b/app/helpers/avatars_helper.rb
index eb3f01138..da39260da 100644
--- a/app/helpers/avatars_helper.rb
+++ b/app/helpers/avatars_helper.rb
@@ -54,7 +54,7 @@ module AvatarsHelper
       elsif user.is_a?(Group)
         group_avatar(options)
       else
-        nil
+        ''
       end
     else
       ''
diff --git a/test/helpers/avatars_helper_test.rb b/test/helpers/avatars_helper_test.rb
index db75e5f7d..c2d2cfa77 100644
--- a/test/helpers/avatars_helper_test.rb
+++ b/test/helpers/avatars_helper_test.rb
@@ -46,9 +46,9 @@ class AvatarsHelperTest < Redmine::HelperTest
     assert_match %r{src="/images/group.png(\?\d+)?"}, avatar(Group.first)
   end
 
-  def test_avatar_with_invalid_arg_should_return_nil
-    assert_nil avatar('jsmith')
-    assert_nil avatar(nil)
+  def test_avatar_with_invalid_arg_should_return_empty_string
+    assert_equal '', avatar('jsmith')
+    assert_equal '', avatar(nil)
   end
 
   def test_avatar_default_size_should_be_24