diff --git a/tools/prune-cache.php b/tools/prune-cache.php
index 43cc9f0..ab30d83 100644
--- a/tools/prune-cache.php
+++ b/tools/prune-cache.php
@@ -132,7 +132,7 @@
     '-H "Accept: application/vnd.github+json"',
     '-H "X-GitHub-Api-Version: 2022-11-28"',
     sprintf('-F ref=%s', $ref),
-    sprintf('/repos/%s/actions/caches?key=%s', $repository, $key),
+    sprintf('/repos/%s/actions/caches?key=%s', $repository, rawurlencode($key)),
     '2>/dev/null',
 ]);