diff --git a/CHANGELOG.md b/CHANGELOG.md
index bfeb713c..fdfd1083 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 ### Added
+- Add `__AVR__` to defines when compiling
 
 ### Changed
 - Move repository from https://github.com/ianfixes/arduino_ci to https://github.com/Arduino-CI/arduino_ci
diff --git a/lib/arduino_ci/cpp_library.rb b/lib/arduino_ci/cpp_library.rb
index 4fbb43d0..42323be2 100644
--- a/lib/arduino_ci/cpp_library.rb
+++ b/lib/arduino_ci/cpp_library.rb
@@ -324,7 +324,7 @@ def build_for_test_with_configuration(test_file, aux_libraries, gcc_binary, ci_g
       executable = Pathname.new("unittest_#{base}.bin").expand_path
       File.delete(executable) if File.exist?(executable)
       arg_sets = []
-      arg_sets << ["-std=c++0x", "-o", executable.to_s, "-DARDUINO=100"]
+      arg_sets << ["-std=c++0x", "-o", executable.to_s, "-DARDUINO=100", "-D__AVR__"]
       if libasan?(gcc_binary)
         arg_sets << [ # Stuff to help with dynamic memory mishandling
           "-g", "-O1",