diff --git a/src/edgecase.rb b/src/edgecase.rb
index a0c782c..71a70b9 100644
--- a/src/edgecase.rb
+++ b/src/edgecase.rb
@@ -51,7 +51,11 @@ module EdgeCase
     end
 
     in_ruby_version("1.9") do
-      AssertionError = MiniTest::Assertion
+      if defined?(MiniTest)
+        AssertionError = MiniTest::Assertion
+      else
+        AssertionError = Test::Unit::AssertionFailedError
+      end
     end
 
     def initialize