diff --git a/src/libexpr/include/nix/expr/nixexpr.hh b/src/libexpr/include/nix/expr/nixexpr.hh
index fd7ed2a67..1fb4d0642 100644
--- a/src/libexpr/include/nix/expr/nixexpr.hh
+++ b/src/libexpr/include/nix/expr/nixexpr.hh
@@ -637,8 +637,8 @@ struct ExprLet : Expr
 struct ExprWith : Expr
 {
     PosIdx pos;
+    uint32_t prevWith;
     Expr *attrs, *body;
-    size_t prevWith;
     ExprWith * parentWith;
     ExprWith(const PosIdx & pos, Expr * attrs, Expr * body)
         : pos(pos)
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index b183f1bbf..81ba5f09f 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -523,6 +523,7 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     prevWith = 0;
     for (curEnv = env.get(), level = 1; curEnv; curEnv = curEnv->up.get(), level++)
         if (curEnv->isWith) {
+            assert(level <= std::numeric_limits<uint32_t>::max());
             prevWith = level;
             break;
         }