diff --git a/tool/lib/minitest/unit.rb b/tool/lib/minitest/unit.rb
index d922a5fc41..2080b26af3 100644
--- a/tool/lib/minitest/unit.rb
+++ b/tool/lib/minitest/unit.rb
@@ -984,6 +984,15 @@ module MiniTest
 
     def location e # :nodoc:
       last_before_assertion = ""
+
+      # debug https://ci.appveyor.com/project/ruby/ruby/builds/26358355/job/27sp43hbqwqetqu0
+      # TODO: remove it
+      if e.backtrace == nil
+        STDERR.puts e.inspect
+        STDERR.puts e.class.ancestors
+        raise e.inspect
+      end
+
       e.backtrace.reverse_each do |s|
         break if s =~ /in .(assert|refute|flunk|pass|fail|raise|must|wont)/
         last_before_assertion = s