diff --git a/src/Monolog/Formatter/HtmlFormatter.php b/src/Monolog/Formatter/HtmlFormatter.php
index 3c4e8479..16a30329 100644
--- a/src/Monolog/Formatter/HtmlFormatter.php
+++ b/src/Monolog/Formatter/HtmlFormatter.php
@@ -92,7 +92,7 @@ class HtmlFormatter extends NormalizerFormatter
         if ($record['context']) {
             $embeddedTable = '<table cellspacing="1" width="100%">';
             foreach ($record['context'] as $key => $value) {
-                $embeddedTable .= $this->addRow($key, $this->convertToString($value));
+                $embeddedTable .= $this->addRow((string)$key, $this->convertToString($value));
             }
             $embeddedTable .= '</table>';
             $output .= $this->addRow('Context', $embeddedTable, false);
@@ -100,7 +100,7 @@ class HtmlFormatter extends NormalizerFormatter
         if ($record['extra']) {
             $embeddedTable = '<table cellspacing="1" width="100%">';
             foreach ($record['extra'] as $key => $value) {
-                $embeddedTable .= $this->addRow($key, $this->convertToString($value));
+                $embeddedTable .= $this->addRow((string)$key, $this->convertToString($value));
             }
             $embeddedTable .= '</table>';
             $output .= $this->addRow('Extra', $embeddedTable, false);