diff --git a/lib/typed_params/comparison.rb b/lib/typed_params/comparison.rb
index ec2b379..76cad7d 100644
--- a/lib/typed_params/comparison.rb
+++ b/lib/typed_params/comparison.rb
@@ -1,12 +1,35 @@
 module TypedParams
   class Comparison
+    # Example usage:
+    # Expected types for arguments indicated on the next line.
+    # Comparison.new(rules_format: RulesFormat, request_body: RequestBody)
+    #
     def initialize(rules_format:, request_body:)
       @rules_format, @request_body = rules_format, request_body
     end
 
+    # Example usage:
+    # comparison.errors =>
+    # Returns nil when there are no errors when comparing the
+    # rules_format to the request_body.
+    # Otherwise, returns an object where each key is the path to the
+    # problematic parameter and whose value describes the problem.
     def errors
       @errors = {}
 
+      # Iterate over each param path and validate the corresponding rule
+      request_body_paths.each do |path|
+        param_with_rule = ParameterWithRule.new(
+          rule:      @rules_format.find(*path),
+          parameter: @request_body.find(*path)
+        )
+
+        error = param_with_rule.validate
+        path_string = path.join("/")
+        @errors[path_string] = param_with_rule.validate if error
+      end
+
+      # Iterate over each rule path and validate the corresponding param
       rule_paths.each do |path|
         param_with_rule = ParameterWithRule.new(
           rule:      @rules_format.find(*path),
@@ -23,9 +46,8 @@ def errors
 
     private
 
-    # List of parameters corresponding to the rules locations.
-    def parameters
-      @parameters ||= paths.map { |path| @request_body.find(*path) }
+    def request_body_paths
+      @request_body.paths
     end
 
     def rule_paths
@@ -41,6 +63,8 @@ def initialize(parameter:, rule:)
     # Return nil if the parameter meets the rule's constraint(s).
     # Otherwise return a tuple conforming to (path_to_param, reason_not_valid).
     def validate
+      return "non_permitted_param" if rule.nil?
+
       if !parameter.value.is_a?(rule.condition)
         "param_must_be_#{rule.condition.name.downcase}"
       end
diff --git a/lib/typed_params/request_body.rb b/lib/typed_params/request_body.rb
index 0e0bc04..c9cad3e 100644
--- a/lib/typed_params/request_body.rb
+++ b/lib/typed_params/request_body.rb
@@ -17,14 +17,8 @@ def paths
     # Returns a Parameter instance when the path points to an existing param.
     # Returns nil otherwise.
     def find(path)
-      param = parameter_at(path)
+      param = @parameters.dig(*path)
       Parameter.new(path, param) if param
     end
-
-    private
-
-    def parameter_at(chunks)
-      @parameters.dig(*chunks)
-    end
   end
 end
diff --git a/lib/typed_params/rules_format.rb b/lib/typed_params/rules_format.rb
index 0c80d92..d076392 100644
--- a/lib/typed_params/rules_format.rb
+++ b/lib/typed_params/rules_format.rb
@@ -13,17 +13,11 @@ def paths
     end
 
     # find(["data", "attributes", "name"])
-    # Raises an error if `rule` not found.
-    # Returns a Rule instance when the path points to an existing param.
+    # Returns a Rule instance when the path points to an existing rule.
+    # Returns nil otherwise.
     def find(path)
-      rule = @rules.dig(*path) || raise(RuleNotFound)
-      Rule.new(path, rule)
-    end
-
-    private
-
-    def rule_at(chunks)
-      @rules.dig(*chunks)
+      rule = @rules.dig(*path)
+      Rule.new(path, rule) if rule
     end
   end
 end
diff --git a/spec/typed_params/comparison_spec.rb b/spec/typed_params/comparison_spec.rb
index 31437e4..43e6c43 100644
--- a/spec/typed_params/comparison_spec.rb
+++ b/spec/typed_params/comparison_spec.rb
@@ -25,10 +25,20 @@
       end
 
       context "when the request body does not adhere to the rules format" do
-        let(:name) { 1_000 }
+        context "when a non-specified parameter is included in the request body" do
+          let(:params) { { name: "M@", email: "matt@zipmark.com" } }
 
-        it "returns an object describing the invalid attribute" do
-          expect(subject).to eq({ "name" => "param_must_be_string" })
+          it "adds an error for the non-whitelisted param" do
+            expect(subject).to eq({ "email" => "non_permitted_param" })
+          end
+        end
+
+        context "when the require parameter is of the wrong type" do
+          let(:name) { 1_000 }
+
+          it "returns an object describing the invalid attribute" do
+            expect(subject).to eq({ "name" => "param_must_be_string" })
+          end
         end
       end
     end
diff --git a/spec/typed_params/request_body_spec.rb b/spec/typed_params/request_body_spec.rb
index a2b90f0..536879d 100644
--- a/spec/typed_params/request_body_spec.rb
+++ b/spec/typed_params/request_body_spec.rb
@@ -23,13 +23,19 @@
       TypedParams::Parameter.new(path, parameters['data']['attributes'])
     end
 
-    context "when the path argument mixes symbols and strings" do
-      let(:path) { [ :data, "attributes" ] }
+    context "when there a rule at the given path" do
+      context "when the path argument mixes symbols and strings" do
+        let(:path) { [ :data, "attributes" ] }
 
-      it { is_expected.to eq paramter }
-    end
+        it { is_expected.to eq paramter }
+      end
+
+      context "when there is no rule at the given path" do
+        let(:path) { %w(foo bar) }
 
-    it { is_expected.to eq paramter }
+        it { is_expected.to eq nil }
+      end
+    end
   end
 
   describe "#paths" do
diff --git a/spec/typed_params/rules_format_spec.rb b/spec/typed_params/rules_format_spec.rb
index 7c75da8..11e9df4 100644
--- a/spec/typed_params/rules_format_spec.rb
+++ b/spec/typed_params/rules_format_spec.rb
@@ -38,9 +38,7 @@
     context "when there is no rule at the given path" do
       let(:path) { %w(foo bar) }
 
-      it "raises an error" do
-        expect { subject }.to raise_error TypedParams::RuleNotFound
-      end
+      it { is_expected.to eq nil }
     end
   end
 end