diff --git a/app/Transformers/BillTransformer.php b/app/Transformers/BillTransformer.php
index b84222281f..b3c4f95681 100644
--- a/app/Transformers/BillTransformer.php
+++ b/app/Transformers/BillTransformer.php
@@ -86,7 +86,7 @@ class BillTransformer extends AbstractTransformer
         foreach ($payDates as $string) {
             $payDatesFormatted[] = Carbon::createFromFormat('!Y-m-d', $string, config('app.timezone'))->toAtomString();
         }
-        $nextExpectedMatch = null;
+        $nextExpectedMatch = '';
         if (null !== ($payDates[0] ?? null)) {
             $nextExpectedMatch = Carbon::createFromFormat('!Y-m-d', $payDates[0], config('app.timezone'))->toAtomString();
         }