diff --git a/lib/rspec/support/method_signature_verifier.rb b/lib/rspec/support/method_signature_verifier.rb
index f3524303..dd6bb971 100644
--- a/lib/rspec/support/method_signature_verifier.rb
+++ b/lib/rspec/support/method_signature_verifier.rb
@@ -84,7 +84,7 @@ def invalid_kw_args_from(given_kw_args)
         def has_kw_args_in?(args)
           Hash === args.last &&
             could_contain_kw_args?(args) &&
-            (args.last.empty? || args.last.keys.any? { |x| x.is_a?(Symbol) })
+            (args.last.empty? || args.last.keys.any? { |x| x.is_a?(Symbol) || x.is_a?(String) })
         end
 
         # Without considering what the last arg is, could it
@@ -365,12 +365,12 @@ def unlimited_args?
       def split_args(*args)
         kw_args = if @signature.has_kw_args_in?(args)
                     last = args.pop
-                    non_kw_args = last.reject { |k, _| k.is_a?(Symbol) }
+                    non_kw_args = last.reject { |k, _| k.is_a?(Symbol) || k.is_a?(String) }
                     if non_kw_args.empty?
                       last.keys
                     else
                       args << non_kw_args
-                      last.select { |k, _| k.is_a?(Symbol) }.keys
+                      last.select { |k, _| k.is_a?(Symbol) || k.is_a(String) }.keys
                     end
                   else
                     []
diff --git a/spec/rspec/support/method_signature_verifier_spec.rb b/spec/rspec/support/method_signature_verifier_spec.rb
index d342425a..e995f73a 100644
--- a/spec/rspec/support/method_signature_verifier_spec.rb
+++ b/spec/rspec/support/method_signature_verifier_spec.rb
@@ -384,17 +384,21 @@ def arity_kw(x, y = {}, z:2); end
               expect(valid?(nil, :a => 1)).to eq(false)
             end
 
-            it 'treats symbols as keyword arguments and the rest as optional argument' do
-              expect(valid?(nil, 'a' => 1)).to eq(true)
-              expect(valid?(nil, 'a' => 1, :z => 3)).to eq(true)
-              expect(valid?(nil, 'a' => 1, :b => 3)).to eq(false)
-              expect(valid?(nil, 'a' => 1, :b => 2, :z => 3)).to eq(false)
+            it 'treats symbols as keyword arguments' do
+              expect(valid?(nil, :z => 3)).to eq(true)
+              expect(valid?(nil, :b => 3)).to eq(false)
+              expect(valid?(nil, :b => 2, :z => 3)).to eq(false)
+            end
+
+            it 'treats string keys as invalid keyword arguments' do
+              expect(valid?(nil, 'a' => 1)).to eq(false)
+              expect(valid?(nil, 'a' => 1, :z => 3)).to eq(false)
             end
 
             it 'mentions the invalid keyword args in the error', :pending => RSpec::Support::Ruby.jruby? && !RSpec::Support::Ruby.jruby_9000? do
               expect(error_for(1, 2, :a => 0)).to eq("Invalid keyword arguments provided: a")
               expect(error_for(1, :a => 0)).to eq("Invalid keyword arguments provided: a")
-              expect(error_for(1, 'a' => 0, :b => 0)).to eq("Invalid keyword arguments provided: b")
+              expect(error_for(1, 'a' => 0, :b => 0)).to eq("Invalid keyword arguments provided: a, b")
             end
 
             it 'describes invalid arity precisely' do
@@ -656,6 +660,41 @@ def arity_required_kw_splat(w, *x, y:, z:, a: 'default'); end
             end
           end
 
+          describe 'a method with only a keyword arg splat' do
+            eval <<-RUBY
+              def arity_kw_arg_splat(**rest); end
+            RUBY
+
+            let(:test_method) { method(:arity_kw_arg_splat) }
+
+            it 'allows undeclared keyword args' do
+              expect(valid?(:x => 1)).to eq(true)
+              expect(valid?(:x => 1, 'y' => 2)).to eq(true)
+            end
+
+            it 'mentions the required kw args and keyword splat in the description' do
+              expect(signature_description).to \
+                eq("any additional keyword args")
+            end
+
+            describe 'with an expectation object' do
+              it 'allows zero non-kw args' do
+                expect(validate_expectation 0).to eq(true)
+                expect(validate_expectation 1).to eq(false)
+                expect(validate_expectation 0, 0).to eq(true)
+                expect(validate_expectation 0, 1).to eq(false)
+              end
+
+              it 'does not match unlimited arguments' do
+                expect(validate_expectation :unlimited_args).to eq(false)
+              end
+
+              it 'matches arbitrary keywords' do
+                expect(validate_expectation :arbitrary_kw_args).to eq(true)
+              end
+            end
+          end
+
           describe 'a method with required keyword arguments and a keyword arg splat' do
             eval <<-RUBY
               def arity_kw_arg_splat(x:, **rest); end
@@ -666,6 +705,7 @@ def arity_kw_arg_splat(x:, **rest); end
             it 'allows extra undeclared keyword args' do
               expect(valid?(:x => 1)).to eq(true)
               expect(valid?(:x => 1, :y => 2)).to eq(true)
+              expect(valid?(:x => 1, :y => 2, 'z' => 3)).to eq(true)
             end
 
             it 'mentions missing required keyword args in the error' do
@@ -730,7 +770,9 @@ def arity_kw_arg_splat(x, **rest); end
             it 'allows a single arg and any number of keyword args' do
               expect(valid?(nil)).to eq(true)
               expect(valid?(nil, :x => 1)).to eq(true)
+              expect(valid?(nil, 'x' => 1)).to eq(true)
               expect(valid?(nil, :x => 1, :y => 2)).to eq(true)
+              expect(valid?(nil, :x => 1, :y => 2, 'z' => 3)).to eq(true)
               expect(valid?(:x => 1)).to eq(true)
               expect(valid?(nil, {})).to eq(true)