diff --git a/test/helper.rb b/test/helper.rb
index de82cd6b5..f283c6a9c 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -8,7 +8,7 @@
 
 $TESTING = true
 # disable minitest/parallel threads
-ENV["MT_CPU"] = 0
+ENV["MT_CPU"] = "0"
 ENV["N"] = "0"
 # Disable any stupid backtrace cleansers
 ENV["BACKTRACE"] = "1"