diff --git a/base/loading.jl b/base/loading.jl
index d0603dc539694..67d2637cf20d1 100644
--- a/base/loading.jl
+++ b/base/loading.jl
@@ -1654,7 +1654,7 @@ require(uuidkey::PkgId) = @lock require_lock _require_prelocked(uuidkey)
 
 const REPL_PKGID = PkgId(UUID("3fa0cd96-eef1-5676-8a61-b3b8758bbffb"), "REPL")
 
-_require_world_age = Ref{UInt}(typemax(UInt))
+const _require_world_age = Ref{UInt}(typemax(UInt))
 
 function _require_prelocked(uuidkey::PkgId, env=nothing)
     if _require_world_age[] != typemax(UInt)