In my build definition, I have some parameters set to "Always Show", like this one:

When I queue a new build, the parameter(s) should display in this dialog:

Instead, I get this error:
Summary: There were 0 failures, 1 errors and 0 warnings loading custom activities and services. Error: Could not load file or assembly 'Microsoft.TeamFoundation.Build.Client, Version=11.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
What is causing this? I have Visual Studio 2013 and TFS Power Tools 2013 installed on Windows 8.1.