diff --git a/Koha/REST/Plugin/Query.pm b/Koha/REST/Plugin/Query.pm
index 01fa0e8cd4..c8ba09e8d4 100644
--- a/Koha/REST/Plugin/Query.pm
+++ b/Koha/REST/Plugin/Query.pm
@@ -512,7 +512,7 @@ sub _validate_operator {
     my ($operator) = @_;
     my %allowed_operators =
         map { $_ => 1 }
-        qw(= != < > <= >= -in -ident -bool -not_bool -or like -like -not_like -between -not_between -not -and -regexp);
+        qw(= != < > <= >= -in -ident -bool -not_bool -or like -like -not_like -between -not_between -not -and -regexp -not_in);
     Koha::Exceptions::REST::Query::InvalidOperator->throw( operator => $operator )
         unless exists $allowed_operators{$operator};
     return;