diff -u -r File/Formula.pm blib/lib/Chemistry/File/Formula.pm
--- File/Formula.pm	2009-05-10 15:37:59.000000000 -0400
+++ blib/lib/Chemistry/File/Formula.pm	2016-12-20 11:06:50.522099876 -0500
@@ -200,7 +200,7 @@
     my $fh = $mol->formula_hash;
     $format =~ s/%%/\\%/g;                          # escape %% with a \
     my $joiner = "";
-    $joiner = $1 if $format =~ s/(?<!\\)%j{(.*?)}//;        # joiner %j{}
+    $joiner = $1 if $format =~ s/(?<!\\)%j\{(.*?)}//;        # joiner %j{}
 
     my @symbols;
     if ($opts{formula_sort}) {