diff --git a/Library/Homebrew/tap.rb b/Library/Homebrew/tap.rb
index ab1749bc2c..bf83e7ee29 100644
--- a/Library/Homebrew/tap.rb
+++ b/Library/Homebrew/tap.rb
@@ -601,7 +601,7 @@ class Tap
     safe_system "git", "-C", path, *args
     git_repository.set_head_origin_auto
 
-    current_upstream_head ||= git_repository.origin_branch_name
+    current_upstream_head ||= T.must(git_repository.origin_branch_name)
 
     new_upstream_head = T.must(git_repository.origin_branch_name)
     return if new_upstream_head == current_upstream_head