diff --git a/test/lib/test/unit/assertions.rb b/test/lib/test/unit/assertions.rb
index ca15bef720..936dcbf9f8 100644
--- a/test/lib/test/unit/assertions.rb
+++ b/test/lib/test/unit/assertions.rb
@@ -490,7 +490,11 @@ EOT
             if 1 < ary.length
               ary[0...-1] = ary[0...-1].map {|str| str.sub(/(?<!\.)\z/, '.') }
             end
-            ary.join("\n")
+            begin
+              ary.join("\n")
+            rescue Encoding::CompatibilityError
+              ary.map(&:b).join("\n")
+            end
           end
         else
           super