diff --git a/lib/redmine/field_format.rb b/lib/redmine/field_format.rb
index bb0185b51..42bfa457b 100644
--- a/lib/redmine/field_format.rb
+++ b/lib/redmine/field_format.rb
@@ -381,8 +381,15 @@ module Redmine
       def validate_single_value(custom_field, value, customized=nil)
         errs = super
         value = value.to_s
-        unless custom_field.regexp.blank? or Regexp.new(custom_field.regexp).match?(value)
-          errs << ::I18n.t('activerecord.errors.messages.invalid')
+        if custom_field.regexp.present?
+          begin
+            unless Regexp.new(custom_field.regexp).match?(value)
+              errs << ::I18n.t('activerecord.errors.messages.invalid')
+            end
+          rescue RegexpError
+            # The invalid regexp is validated in CustomField#validate_custom_field,
+            # so we can skip validation here
+          end
         end
         if custom_field.min_length && value.length < custom_field.min_length
           errs << ::I18n.t('activerecord.errors.messages.too_short', :count => custom_field.min_length)