diff --git a/pkgs/development/interpreters/tcl/generic.nix b/pkgs/development/interpreters/tcl/generic.nix
index e1925821e5a..011b3a125ef 100644
--- a/pkgs/development/interpreters/tcl/generic.nix
+++ b/pkgs/development/interpreters/tcl/generic.nix
@@ -19,13 +19,14 @@ stdenv.mkDerivation rec {
   '';
 
   configureFlags = [
+    "--enable-threads"
     # Note: using $out instead of $man to prevent a runtime dependency on $man.
     "--mandir=${placeholder "out"}/share/man"
     "--enable-man-symlinks"
     # Don't install tzdata because NixOS already has a more up-to-date copy.
     "--with-tzdata=no"
     "tcl_cv_strtod_unbroken=ok"
-  ];
+  ] ++ stdenv.lib.optional stdenv.is64bit "--enable-64bit";
 
   enableParallelBuilding = true;