diff --git a/pkgs/development/interpreters/tcl/generic.nix b/pkgs/development/interpreters/tcl/generic.nix
index a472ccaafbf..e1925821e5a 100644
--- a/pkgs/development/interpreters/tcl/generic.nix
+++ b/pkgs/development/interpreters/tcl/generic.nix
@@ -15,15 +15,18 @@ stdenv.mkDerivation rec {
   setOutputFlags = false;
 
   preConfigure = ''
-    # Note: using $out instead of $man to prevent a runtime dependency on $man.
-    configureFlagsArray+=(--mandir=$out/share/man --enable-man-symlinks)
-
-    # Don't install tzdata because NixOS already has a more up-to-date copy.
-    configureFlagsArray+=(--with-tzdata=no)
-
     cd unix
   '';
 
+  configureFlags = [
+    # 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"
+  ];
+
   enableParallelBuilding = true;
 
   postInstall = ''