diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index d5086af2..960c65bf 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -324,7 +324,7 @@ class Backend {
                 {},
                 options
             );
-            if (definitions.length > 0) {
+            if (definitions.length > 0 && sourceLength > 0) {
                 dictTermsSort(definitions);
                 const {expression, reading} = definitions[0];
                 const source = text.substring(0, sourceLength);