diff --git a/exercise-solutions/multi-threaded-mailbox/Cargo.lock b/exercise-solutions/multi-threaded-mailbox/Cargo.lock
index 176852c7..f5353a3d 100644
--- a/exercise-solutions/multi-threaded-mailbox/Cargo.lock
+++ b/exercise-solutions/multi-threaded-mailbox/Cargo.lock
@@ -4,7 +4,7 @@ version = 3
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.61"
+version = "1.0.63"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7b368fba921b0dce7e60f5e04ec15e565b3303972b42bcfde1d0713b881959eb"
 dependencies = [