Index: branches/release-33/lib/MT/Template/ContextHandlers.pm
===================================================================
--- branches/release-33/lib/MT/Template/ContextHandlers.pm (revision 1772)
+++ branches/release-33/lib/MT/Template/ContextHandlers.pm (revision 1774)
@@ -2315,4 +2315,10 @@
     my $use_ssi = $blog && $blog->include_system
         && ($arg->{ssi} || $tmpl->include_with_ssi) ? 1 : 0;
+    if ($use_ssi) {
+        # disable SSI for templates that are system templates;
+        # easiest way to determine this is from the variable
+        # space setting.
+        $use_ssi = 0 if $ctx->var('system_template');
+    }
 
     # Try to read from cache