diff --git a/Mac/BuildScript/build-installer.py b/Mac/BuildScript/build-installer.py
index b5bffaff4ed0dc..50b55f9cd1da84 100755
--- a/Mac/BuildScript/build-installer.py
+++ b/Mac/BuildScript/build-installer.py
@@ -271,10 +271,11 @@ def library_recipes():
             tk_patches = [ ]
 
 
+        base_url = "https://prdownloads.sourceforge.net/tcl/{what}{version}-src.tar.gz"
         result.extend([
           dict(
               name="Tcl %s"%(tcl_tk_ver,),
-              url="ftp://ftp.tcl.tk/pub/tcl//tcl8_6/tcl%s-src.tar.gz"%(tcl_tk_ver,),
+              url=base_url.format(what="tcl", version=tcl_tk_ver),
               checksum=tcl_checksum,
               buildDir="unix",
               configure_pre=[
@@ -291,7 +292,7 @@ def library_recipes():
               ),
           dict(
               name="Tk %s"%(tcl_tk_ver,),
-              url="ftp://ftp.tcl.tk/pub/tcl//tcl8_6/tk%s-src.tar.gz"%(tcl_tk_ver,),
+              url=base_url.format(what="tk", version=tcl_tk_ver),
               checksum=tk_checksum,
               patches=tk_patches,
               buildDir="unix",