diff --git a/Gemfile.lock b/Gemfile.lock
index 0280b53..e655c2d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -14,7 +14,7 @@ PATH
     flack (0.11.0)
       flor (~> 0.11)
       httpclient (~> 2.8)
-      rack (~> 1.6)
+      rack (>= 1.6, < 3.0)
       sequel (~> 4)
 
 GEM
@@ -30,7 +30,7 @@ GEM
     jdbc-sqlite3 (3.8.11.2)
     munemo (1.0.1)
     raabro (1.1.5)
-    rack (1.6.8)
+    rack (2.2.3.1)
     rspec (3.4.0)
       rspec-core (~> 3.4.0)
       rspec-expectations (~> 3.4.0)
diff --git a/flack.gemspec b/flack.gemspec
index 77240d7..cb26446 100644
--- a/flack.gemspec
+++ b/flack.gemspec
@@ -31,7 +31,7 @@ A web front-end to the flor workflow engine
   flor_version = s.version.to_s.split('.').take(2).join('.')
   s.add_runtime_dependency 'flor', "~> #{flor_version}"
 
-  s.add_runtime_dependency 'rack', '~> 1.6'
+  s.add_runtime_dependency 'rack', '>= 1.6', '< 3.0'
   s.add_runtime_dependency 'sequel', '~> 4'
   s.add_runtime_dependency 'httpclient', '~> 2.8'