diff --git a/lib/persistent_open_struct.rb b/lib/persistent_open_struct.rb
index 0a21789..21b0704 100644
--- a/lib/persistent_open_struct.rb
+++ b/lib/persistent_open_struct.rb
@@ -68,11 +68,11 @@ def modifiable
   def method_missing(mid, *args) # :nodoc:
     mname = mid.id2name
     len = args.length
-    if mname.chomp!('=')
+    if mname.end_with?('=')
       if len != 1
         raise ArgumentError, "wrong number of arguments (#{len} for 1)", caller(1)
       end
-      modifiable[new_ostruct_member(mname)] = args[0]
+      modifiable[new_ostruct_member(mname.chomp('='))] = args[0]
     elsif len == 0
       @table[mid]
     else
diff --git a/persistent_open_struct.gemspec b/persistent_open_struct.gemspec
index 6fe3275..ff56932 100644
--- a/persistent_open_struct.gemspec
+++ b/persistent_open_struct.gemspec
@@ -19,8 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.required_ruby_version = '>= 2.1.0'
-  spec.add_development_dependency "bundler", "~> 1.6"
-  spec.add_development_dependency 'rake', '~> 10.4', '>= 10.4.2'
+  spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'minitest', '~> 5.4', '>= 5.4.3'
   spec.add_development_dependency 'benchmark-ips', '~> 2.6'
 end
diff --git a/test/test_persistent_open_struct.rb b/test/test_persistent_open_struct.rb
index 0b76f15..7c88d5c 100644
--- a/test/test_persistent_open_struct.rb
+++ b/test/test_persistent_open_struct.rb
@@ -76,7 +76,7 @@ def test_delete_field
     assert_respond_to(o, :foobar)
     assert_respond_to(o, :foobar=)
     o.delete_field(:foobar)
-    assert_equal(nil, o.foobar)
+    assert_nil(o.foobar)
     assert_respond_to(o, :foobar)
     assert_respond_to(o, :foobar=)
   end