diff --git a/modules/document_repository/jsx/uploadForm.js b/modules/document_repository/jsx/uploadForm.js
index 7785e00d445..beb5b893ad3 100644
--- a/modules/document_repository/jsx/uploadForm.js
+++ b/modules/document_repository/jsx/uploadForm.js
@@ -240,6 +240,12 @@ class DocUploadForm extends Component {
                     'error'
                 );
               }
+              if (resp.status == 400) {
+                swal.fire('Something went wrong',
+                    JSON.parse(resp.response).message,
+                    'error'
+                );
+              }
           }
         }).catch((error) => {
           console.error(error);
diff --git a/modules/document_repository/php/files.class.inc b/modules/document_repository/php/files.class.inc
index 3eb5efc6a97..7dd50b224f1 100644
--- a/modules/document_repository/php/files.class.inc
+++ b/modules/document_repository/php/files.class.inc
@@ -316,20 +316,18 @@ class Files extends \NDB_Page
         if (!is_array($body)) {
             throw new \LorisException("Expected parsed body to be an array");
         }
-
-        $category = $body['category']; // required
-        if (!\Utility::valueIsPositiveInteger($category)) {
-            // $category is a string representation of an ID, and so should be
-            // at least equal to zero.
-            return new \LORIS\Http\Response\JSON\BadRequest(self::BAD_CATEGORY);
-        }
-
         $uploadedFiles = $request->getUploadedFiles()['files'] ?? null;
 
         if (is_null($uploadedFiles)) {
             header("HTTP/1.1 413 Too_large");
             exit;
         }
+        $category = $body['category']; // required
+        if (!\Utility::valueIsPositiveInteger(strval($category))) {
+            // $category is a string representation of an ID, and so should be
+            // at least equal to zero.
+            return new \LORIS\Http\Response\JSON\BadRequest(self::BAD_CATEGORY);
+        }
 
         $uploadedFiles = is_array($uploadedFiles)
             ? $uploadedFiles : [$uploadedFiles];