Fix `$rake_root_fiber` checking.

https://github.com/mruby/mruby/pull/4101#pullrequestreview-187752352

1 files changed, 1 insertions, 1 deletions

diff --git a/minirake b/minirake
index 86c42c999..cd2d85854 100755
--- a/minirake
+++ b/minirake
@@ -483,7 +483,7 @@ class RakeApp
   def run
     handle_options
 
-    if $rake_root_fiber
+    unless $rake_root_fiber
       require 'fiber'
       $rake_root_fiber = Fiber.current
     end