diff --git a/src/security/tpm/back-end-file.hpp b/src/security/tpm/back-end-file.hpp
index db58907..5609cba 100644
--- a/src/security/tpm/back-end-file.hpp
+++ b/src/security/tpm/back-end-file.hpp
@@ -134,7 +134,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace tpm