diff --git a/src/solvers/flattening/bv_utils.h b/src/solvers/flattening/bv_utils.h
index 35a64eec59b..1b988cda0c6 100644
--- a/src/solvers/flattening/bv_utils.h
+++ b/src/solvers/flattening/bv_utils.h
@@ -179,17 +179,17 @@ class bv_utilst
   static bvt
   extension(const bvt &bv, std::size_t new_size, representationt rep);
 
-  bvt sign_extension(const bvt &bv, std::size_t new_size)
+  static bvt sign_extension(const bvt &bv, std::size_t new_size)
   {
     return extension(bv, new_size, representationt::SIGNED);
   }
 
-  bvt zero_extension(const bvt &bv, std::size_t new_size)
+  static bvt zero_extension(const bvt &bv, std::size_t new_size)
   {
     return extension(bv, new_size, representationt::UNSIGNED);
   }
 
-  bvt zeros(std::size_t new_size) const
+  static bvt zeros(std::size_t new_size)
   {
     bvt result;
     result.resize(new_size, const_literal(false));