diff --git a/pkgs/development/interpreters/tcl/8.5.nix b/pkgs/development/interpreters/tcl/8.5.nix
index 6a8e2391751c..5167475dc3ce 100644
--- a/pkgs/development/interpreters/tcl/8.5.nix
+++ b/pkgs/development/interpreters/tcl/8.5.nix
@@ -12,5 +12,9 @@ callPackage ./generic.nix (
       url = "mirror://sourceforge/tcl/tcl${version}-src.tar.gz";
       sha256 = "066vlr9k5f44w9gl9382hlxnryq00d5p6c7w5vq1fgc7v9b49w6k";
     };
+
+    extraPatch = ''
+      substituteInPlace 'generic/tclInt.h' --replace-fail 'typedef int ptrdiff_t;' ""
+    '';
   }
 )
diff --git a/pkgs/development/interpreters/tcl/generic.nix b/pkgs/development/interpreters/tcl/generic.nix
index 86bc54b9ceb3..6c0c6ffb4aa6 100644
--- a/pkgs/development/interpreters/tcl/generic.nix
+++ b/pkgs/development/interpreters/tcl/generic.nix
@@ -12,6 +12,7 @@
   release,
   version,
   src,
+  extraPatch ? "",
   ...
 }:
 
@@ -27,13 +28,15 @@ let
 
     setOutputFlags = false;
 
-    postPatch = ''
-      substituteInPlace library/clock.tcl \
-        --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo" \
-        --replace "/usr/share/lib/zoneinfo" "" \
-        --replace "/usr/lib/zoneinfo" "" \
-        --replace "/usr/local/etc/zoneinfo" ""
-    '';
+    postPatch =
+      ''
+        substituteInPlace library/clock.tcl \
+          --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo" \
+          --replace "/usr/share/lib/zoneinfo" "" \
+          --replace "/usr/lib/zoneinfo" "" \
+          --replace "/usr/local/etc/zoneinfo" ""
+      ''
+      + extraPatch;
 
     nativeBuildInputs = lib.optionals (lib.versionAtLeast version "9.0") [
       # Only used to detect the presence of zlib. Could be replaced with a stub.