diff --git a/lib/manager/docker/extract.js b/lib/manager/docker/extract.js
index ac2c7cab88876e8478ce3516cb70b5c4a6006e7e..2a12eaac68ecdd73843eaf9e5b355650f5ba411b 100644
--- a/lib/manager/docker/extract.js
+++ b/lib/manager/docker/extract.js
@@ -4,6 +4,10 @@ module.exports = {
 };
 
 function groupedRegex(string, pattern) {
+  // istanbul ignore if
+  if (!string) {
+    return [];
+  }
   const matches = string.match(new RegExp(pattern.source, pattern.flags));
   if (!matches) {
     return [];