diff --git a/src/adjacent.hpp b/src/adjacent.hpp
index 4aded01..29d7567 100644
--- a/src/adjacent.hpp
+++ b/src/adjacent.hpp
@@ -115,7 +115,7 @@
   }
 
   uint64_t
-  getFaceId()
+  getFaceId() const
   {
     return m_faceId;
   }