diff --git a/src/hei_isolation_data.hpp b/src/hei_isolation_data.hpp
index df07e05..93ec5b8 100644
--- a/src/hei_isolation_data.hpp
+++ b/src/hei_isolation_data.hpp
@@ -45,7 +45,7 @@
     }
 
     /** @brief Allows access to the signature list. */
-    const std::vector<Signature>& getSignatureList()
+    const std::vector<Signature>& getSignatureList() const
     {
         return iv_sigLists;
     }