diff --git a/src/BookSearch/BookSearch.php b/src/BookSearch/BookSearch.php
index 69fba4e..98a7045 100644
--- a/src/BookSearch/BookSearch.php
+++ b/src/BookSearch/BookSearch.php
@@ -83,7 +83,7 @@ class BookSearch
 
         if($queryField == 'mediaType' and $queryText=='noCDS'){
             $queryText='-mediaType:CDS';
-        } else if (strlen($queryField) > 0) {
+        } else if (isset($queryField) && strlen($queryField) > 0) {
             $queryText = sprintf('%s:"%s"', $queryField, $queryText);
         }