diff --git a/src/CacheAdapter.php b/src/CacheAdapter.php
index 38dabef..bef5437 100644
--- a/src/CacheAdapter.php
+++ b/src/CacheAdapter.php
@@ -326,7 +326,7 @@ public function checksum(string $path, Config $config): string
         $algo = $config->get('checksum_algo');
 
         if (isset($algo) && !is_string($algo)) {
-            throw new ErrorException('"checksum_algo" $config must be a string.');
+            throw new ErrorException('"checksum_algo" in $config must be a string.');
         }
 
         $metadataKey = isset($algo) ? 'checksum_' . $algo : 'checksum';