diff --git a/lib/debug/session.rb b/lib/debug/session.rb
index c95edd57b..a14804440 100644
--- a/lib/debug/session.rb
+++ b/lib/debug/session.rb
@@ -2231,7 +2231,7 @@ def self.step_in &b
     # depend on the file system. So this check is only roughly estimation.
 
     def self.compare_path(a, b)
-      a.downcase == b.downcase
+      a&.downcase == b&.downcase
     end
   else
     def self.compare_path(a, b)