diff --git a/src/util/lp/permutation_matrix.h b/src/util/lp/permutation_matrix.h
index 27d971415..401f5583a 100644
--- a/src/util/lp/permutation_matrix.h
+++ b/src/util/lp/permutation_matrix.h
@@ -123,7 +123,7 @@ namespace lean {
 
         unsigned size() const { return static_cast<unsigned>(m_rev.size()); }
 
-        unsigned * values() const { return m_permutation; }
+        const unsigned * values() const { return m_permutation.data(); }
     }; // end of the permutation class
 
 #ifdef LEAN_DEBUG