diff --git a/extras/fixture/rakefile_helper.rb b/extras/fixture/rakefile_helper.rb
index 5aa8e56..c45b239 100644
--- a/extras/fixture/rakefile_helper.rb
+++ b/extras/fixture/rakefile_helper.rb
@@ -53,7 +53,7 @@ module RakefileHelpers
     defines = if $cfg['compiler']['defines']['items'].nil?
                 ''
               else
-                squash($cfg['compiler']['defines']['prefix'], $cfg['compiler']['defines']['items'] + ['UNITY_OUTPUT_CHAR=UnityOutputCharSpy_OutputChar'])
+                squash($cfg['compiler']['defines']['prefix'], $cfg['compiler']['defines']['items'] + ['UNITY_OUTPUT_CHAR=UnityOutputCharSpy_OutputChar'] + ['UNITY_OUTPUT_CHAR_HEADER_DECLARATION=UnityOutputCharSpy_OutputChar\(int\)'])
               end
     options  = squash('', $cfg['compiler']['options'])
     includes = squash($cfg['compiler']['includes']['prefix'], $cfg['compiler']['includes']['items'])