diff --git a/src/js/plugins/previewThumbnails.js b/src/js/plugins/previewThumbnails.js
index bd7a6bbd..90e6226d 100644
--- a/src/js/plugins/previewThumbnails.js
+++ b/src/js/plugins/previewThumbnails.js
@@ -148,7 +148,10 @@ class PreviewThumbnails {
 
                 // If the URLs don't start with '/', then we need to set their relative path to be the location of the VTT file
                 // If the URLs do start with '/', then they obviously don't need a prefix, so it will remain blank
-                if (!thumbnail.frames[0].text.startsWith('/')) {
+                // If the thumbnail URLs start with with none of '/', 'http://' or 'https://', then we need to set their relative path to be the location of the VTT file
+                if (!thumbnail.frames[0].text.startsWith('/') &&
+                    !thumbnail.frames[0].text.startsWith('http://') &&
+                    !thumbnail.frames[0].text.startsWith('https://')) {
                     thumbnail.urlPrefix = url.substring(0, url.lastIndexOf('/') + 1);
                 }