diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index c8b6046bb8d6..98feea96f8c4 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -550,6 +550,7 @@ impl ThreadId {
             // If we somehow use up all our bits, panic so that we're not
             // covering up subtle bugs of IDs being reused.
             if COUNTER == ::u64::MAX {
+                GUARD.unlock();
                 panic!("failed to generate unique thread ID: bitspace exhausted");
             }