diff --git a/lib/rake/backtrace.rb b/lib/rake/backtrace.rb
index 439255d78b..c9b0a7c089 100644
--- a/lib/rake/backtrace.rb
+++ b/lib/rake/backtrace.rb
@@ -1,6 +1,6 @@
 module Rake
   module Backtrace # :nodoc: all
-    SYS_KEYS  = RbConfig::CONFIG.keys.grep(/(prefix|libdir)/)
+    SYS_KEYS  = RbConfig::CONFIG.keys.grep(/(?:prefix|libdir)\z/)
     SYS_PATHS = RbConfig::CONFIG.values_at(*SYS_KEYS).uniq +
       [ File.join(File.dirname(__FILE__), "..") ]