diff --git a/.travis.yml b/.travis.yml
index 9e2e6e1614..dca6c0c488 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -31,7 +31,7 @@ env:
 # Run the NUnit tests
 script:
  - travis_retry make all-dependencies
- - make all
+ - make all SDK="-sdk:4.0"
  - make check
  - make check-scripts
  - make test
diff --git a/Makefile b/Makefile
index 3223a2b5bc..17a7f27fdf 100644
--- a/Makefile
+++ b/Makefile
@@ -42,7 +42,8 @@
 
 ############################## TOOLCHAIN ###############################
 #
-CSC         = mcs -sdk:4.0
+SDK         ?=
+CSC         = mcs $(SDK)
 CSFLAGS     = -nologo -warn:4 -codepage:utf8 -unsafe -warnaserror
 DEFINE      = TRACE
 COMMON_LIBS = System.dll System.Core.dll System.Data.dll System.Data.DataSetExtensions.dll System.Drawing.dll System.Xml.dll thirdparty/download/ICSharpCode.SharpZipLib.dll thirdparty/download/FuzzyLogicLibrary.dll thirdparty/download/Mono.Nat.dll thirdparty/download/MaxMind.Db.dll thirdparty/download/MaxMind.GeoIP2.dll thirdparty/download/Eluant.dll thirdparty/download/SmarIrc4net.dll