C# backend error with filters (fcfg is nil in dotnet.lua)

So I was changing some code I have to use per-configuration filters like the following:

  filter { "system:windows", "architecture:x86" }
    files { "CSharp/i686-pc-win32-msvc/**.cs" }

  filter { "system:windows", "architecture:x86_64" }
    files { "CSharp/x86_64-pc-win32-msvc/**.cs" }

  filter { "system:macosx", "architecture:x86" }
    files { "CSharp/i686-apple-darwin12.4.0/**.cs" }

  filter { "system:macosx", "architecture:x86_64" }
    files { "CSharp/x86_64-apple-darwin12.4.0/**.cs" }

And now I get:

stack traceback:
    /Users/joao/Dev/premake-core/src/tools/dotnet.lua:35: in function 'fileinfo'
    ...ao/Dev/premake-core/src/actions/make/make_csharp.lua:294: in function 'selector'
    ...ao/Dev/premake-core/src/actions/make/make_csharp.lua:116: in function 'onleaf'
    /Users/joao/Dev/premake-core/src/base/tree.lua:241: in function 'donode'
    /Users/joao/Dev/premake-core/src/base/tree.lua:251: in function 'dochildren'
    /Users/joao/Dev/premake-core/src/base/tree.lua:235: in function 'donode'
    /Users/joao/Dev/premake-core/src/base/tree.lua:251: in function 'dochildren'
    /Users/joao/Dev/premake-core/src/base/tree.lua:235: in function 'donode'
    /Users/joao/Dev/premake-core/src/base/tree.lua:251: in function 'dochildren'
    /Users/joao/Dev/premake-core/src/base/tree.lua:266: in function 'traverse'
    ...ao/Dev/premake-core/src/actions/make/make_csharp.lua:114: in function 'listsources'
    ...ao/Dev/premake-core/src/actions/make/make_csharp.lua:292: in function '?'
    ...Users/joao/Dev/premake-core/src/base/_foundation.lua:108: in function 'callArray'
    ...ao/Dev/premake-core/src/actions/make/make_csharp.lua:57: in function 'callback'
    /Users/joao/Dev/premake-core/src/base/premake.lua:155: in function 'fn'
    /Users/joao/Dev/premake-core/src/base/premake.lua:49: in function 'capture'
    /Users/joao/Dev/premake-core/src/base/premake.lua:153: in function 'generate'
    ...ers/joao/Dev/premake-core/src/actions/make/_make.lua:46: in function 'onProject'
    /Users/joao/Dev/premake-core/src/base/action.lua:108: in function 'call'
    /Users/joao/Dev/premake-core/src/_premake_main.lua:330: in function '?'
    ...Users/joao/Dev/premake-core/src/base/_foundation.lua:108: in function 'callArray'
    /Users/joao/Dev/premake-core/src/_premake_main.lua:371: in function </Users/joao/Dev/premake-core/src/_premake_main.lua:370>
Error: /Users/joao/Dev/premake-core/src/tools/dotnet.lua:38: attempt to index local 'fcfg' (a nil value)

If I remove the last filter block then the error goes away.

Reproduce the failure with the per-configuration filter blocks shown in the issue, then inspect src/tools/dotnet.lua around lines 35-38 and the callers in src/actions/make/make_csharp.lua. Trace why the matching configuration is absent and verify that C# makefile generation completes without indexing a nil configuration.