diff --git a/run.rb b/run.rb
index 98c8c55..21516e2 100644
--- a/run.rb
+++ b/run.rb
@@ -29,7 +29,7 @@ def start_reporting_process(reader)
         next
       end
 
-      seen[cov_hash] = Base64.strict_decode64(encoded_bytes)
+      seen[cov_hash] = Base64.strict_decode64(encoded_bytes).freeze
       puts(seen)
     end
   end