diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index 1f2a6aebc0..b3dad0ee89 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -579,7 +579,8 @@ module Gem
 
     def self.last_date(path)
       return unless $vcs
-      return unless time = $vcs.get_revisions(path)[2]
+      time = $vcs.get_revisions(path)[2] rescue return
+      return unless time
       time.strftime("%Y-%m-%d")
     end
 
diff --git a/tool/vcs.rb b/tool/vcs.rb
index 6e86cfb07e..dbde58545e 100644
--- a/tool/vcs.rb
+++ b/tool/vcs.rb
@@ -43,8 +43,8 @@ class VCS
   def get_revisions(path)
     path = relative_to(path)
     last, changed, modified, *rest = Dir.chdir(@srcdir) {self.class.get_revisions(path)}
-    last or raise "last revision not found"
-    changed or raise "changed revision not found"
+    last or raise VCS::NotFoundError, "last revision not found"
+    changed or raise VCS::NotFoundError, "changed revision not found"
     modified &&= Time.parse(modified)
     return last, changed, modified, *rest
   end