diff --git a/SPEC.rdoc b/SPEC.rdoc
index e343bc430..ed5d98244 100644
--- a/SPEC.rdoc
+++ b/SPEC.rdoc
@@ -130,7 +130,7 @@ There are the following restrictions:
 * There may be a valid early hints callback in <tt>rack.early_hints</tt>
 * The <tt>REQUEST_METHOD</tt> must be a valid token.
 * The <tt>SCRIPT_NAME</tt>, if non-empty, must start with <tt>/</tt>
-* The <tt>PATH_INFO</tt>, if provided, must be a valid request target.
+* The <tt>PATH_INFO</tt>, if provided, must be a valid request target or an empty string.
   * Only <tt>OPTIONS</tt> requests may have <tt>PATH_INFO</tt> set to <tt>*</tt> (asterisk-form).
   * Only <tt>CONNECT</tt> requests may have <tt>PATH_INFO</tt> set to an authority (authority-form). Note that in HTTP/2+, the authority-form is not a valid request target.
   * <tt>CONNECT</tt> and <tt>OPTIONS</tt> requests must not have <tt>PATH_INFO</tt> set to a URI (absolute-form).
diff --git a/config/external.yaml b/config/external.yaml
index dfb0fc67f..7ca421b10 100644
--- a/config/external.yaml
+++ b/config/external.yaml
@@ -11,3 +11,6 @@ roda:
 grape:
   url: https://github.com/ruby-grape/grape
   command: bundle exec rspec --exclude-pattern=spec/integration/**/*_spec.rb
+sinatra:
+  url: https://github.com/sinatra/sinatra
+  command: bundle exec rake test
diff --git a/lib/rack/lint.rb b/lib/rack/lint.rb
index 1ab3c271b..d1720884d 100644
--- a/lib/rack/lint.rb
+++ b/lib/rack/lint.rb
@@ -361,7 +361,7 @@ def check_environment(env)
           raise LintError, "SCRIPT_NAME must start with /"
         end
 
-        ## * The <tt>PATH_INFO</tt>, if provided, must be a valid request target.
+        ## * The <tt>PATH_INFO</tt>, if provided, must be a valid request target or an empty string.
         if env.include?(PATH_INFO)
           case env[PATH_INFO]
           when REQUEST_PATH_ASTERISK_FORM
@@ -381,6 +381,8 @@ def check_environment(env)
             end
           when REQUEST_PATH_ORIGIN_FORM
             ##   * Otherwise, <tt>PATH_INFO</tt> must start with a <tt>/</tt> and must not include a fragment part starting with '#' (origin-form).
+          when ""
+            # Empty string is okay.
           else
             raise LintError, "PATH_INFO must start with a '/' and must not include a fragment part starting with '#' (origin-form)"
           end
diff --git a/test/spec_lint.rb b/test/spec_lint.rb
index 573eb4153..efa5de870 100644
--- a/test/spec_lint.rb
+++ b/test/spec_lint.rb
@@ -310,6 +310,10 @@ def result.name
       message.must_include('response array has 4 elements instead of 3')
   end
 
+  it "accepts empty PATH_INFO" do
+    Rack::Lint.new(valid_app).call(env("PATH_INFO" => "")).first.must_equal 200
+  end
+
   it "notices request-target asterisk form errors" do
     # A non-empty PATH_INFO starting with something other than / has
     # implications for Rack::Request#path and methods downstream from