diff --git a/ext/-test-/hash/delete.c b/ext/-test-/hash/delete.c
index 1a7c373b97..10d4aec6e7 100644
--- a/ext/-test-/hash/delete.c
+++ b/ext/-test-/hash/delete.c
@@ -1,5 +1,7 @@
 #include "ruby.h"
 
+extern VALUE rb_hash_delete_entry(VALUE hash, VALUE key);
+
 static VALUE
 hash_delete(VALUE hash, VALUE key)
 {