diff --git a/opac/unapi b/opac/unapi
index 62ae2d7849..0212e87a40 100755
--- a/opac/unapi
+++ b/opac/unapi
@@ -106,13 +106,22 @@ my $format_info = {
 my $id     = $cgi->param('id');
 my $format = $cgi->param('format');
 
-if (not defined $format) {
-    emit_formats($id, $format_to_stylesheet_map, $format_info, $cgi);
-} elsif ($id) {
+my $biblionumber;
+
+if ( $id && $id =~ /koha:biblionumber:(\d+)/ ) {
+    $biblionumber = $1;
+} else {
+    $id = undef;
+}
+
+unless ( $format && $format_info->{$format} ) {
+    $format = undef;
+}
+
+if ( $id && $format ) {
 
     # koha:biblionumber:0152018484
-    if ($id =~ /koha:biblionumber:(\d+)/) {
-        my $biblionumber = $1;
+    if ($biblionumber) {
 
         my $content;
 
@@ -146,6 +155,9 @@ if (not defined $format) {
         print $cgi->header( -status => '404 record not found');
         exit 0;
     }
+} elsif ( $id && not defined $format ) {
+    emit_formats( $id, $format_to_stylesheet_map, $format_info, $cgi );
+
 } else {
     # supplied a format but no id - caller is doing it wrong
     print $cgi->header( -status => '400 bad request - if you specify format, must specify id');