Index: /branches/release-39/lib/MT/Template/ContextHandlers.pm
===================================================================
--- /branches/release-39/lib/MT/Template/ContextHandlers.pm (revision 2442)
+++ /branches/release-39/lib/MT/Template/ContextHandlers.pm (revision 2448)
@@ -5511,5 +5511,5 @@
         $val = $existing . $val;
     }
-    elsif ( defined($existing) && ( my $op = $args->{op} ) ) {
+    elsif ( $existing ne '' && ( my $op = $args->{op} ) ) {
         $val = _math_operation($ctx, $op, $existing, $val);
     }