diff --git a/lib/secure_headers/headers/content_security_policy.rb b/lib/secure_headers/headers/content_security_policy.rb
index b1873179..efee3428 100644
--- a/lib/secure_headers/headers/content_security_policy.rb
+++ b/lib/secure_headers/headers/content_security_policy.rb
@@ -49,15 +49,16 @@ def value
     # frame-src is deprecated, child-src is being implemented. They are
     # very similar and in most cases, the same value can be used for both.
     def normalize_child_frame_src
-      Kernel.warn("#{Kernel.caller.first}: [DEPRECATION] :frame_src is deprecated, use :child_src instead. Provided: #{@config[:frame_src]}") if @config[:frame_src]
+      if @config[:frame_src] && @config[:child_src] && @config[:frame_src] != @config[:child_src]
+        Kernel.warn("#{Kernel.caller.first}: [DEPRECATION] both :child_src and :frame_src supplied and do not match. This can lead to inconsistent behavior across browsers.")
+      elsif @config[:frame_src]
+        Kernel.warn("#{Kernel.caller.first}: [DEPRECATION] :frame_src is deprecated, use :child_src instead. Provided: #{@config[:frame_src]}.")
+      end
 
-      child_src = @config[:child_src] || @config[:frame_src]
-      if child_src
-        if supported_directives.include?(:child_src)
-          @config[:child_src] = child_src
-        else
-          @config[:frame_src] = child_src
-        end
+      if supported_directives.include?(:child_src)
+        @config[:child_src] = @config[:child_src] || @config[:frame_src]
+      else
+        @config[:frame_src] = @config[:frame_src] || @config[:child_src]
       end
     end
 
diff --git a/spec/lib/secure_headers/headers/content_security_policy_spec.rb b/spec/lib/secure_headers/headers/content_security_policy_spec.rb
index dbbf387a..1e30f62b 100644
--- a/spec/lib/secure_headers/headers/content_security_policy_spec.rb
+++ b/spec/lib/secure_headers/headers/content_security_policy_spec.rb
@@ -86,6 +86,27 @@ module SecureHeaders
         expect(csp.value).to eq("default-src example.org")
       end
 
+      it "emits a warning when using frame-src" do
+        expect(Kernel).to receive(:warn).with(/:frame_src is deprecated, use :child_src instead./)
+        ContentSecurityPolicy.new(default_src: %w('self'), frame_src: %w('self')).value
+      end
+
+      it "emits a warning when child-src and frame-src are supplied but are not equal" do
+        expect(Kernel).to receive(:warn).with(/both :child_src and :frame_src supplied and do not match./)
+        ContentSecurityPolicy.new(default_src: %w('self'), child_src: %w(child-src.com), frame_src: %w(frame-src,com)).value
+      end
+
+      it "will still set inconsistent child/frame-src values to be less surprising" do
+        expect(Kernel).to receive(:warn).at_least(:once)
+        firefox = ContentSecurityPolicy.new({default_src: %w('self'), child_src: %w(child-src.com), frame_src: %w(frame-src,com)}, USER_AGENTS[:firefox]).value
+        firefox_transitional = ContentSecurityPolicy.new({default_src: %w('self'), child_src: %w(child-src.com), frame_src: %w(frame-src,com)}, USER_AGENTS[:firefox46]).value
+        expect(firefox).not_to eq(firefox_transitional)
+        expect(firefox).to match(/frame-src/)
+        expect(firefox).not_to match(/child-src/)
+        expect(firefox_transitional).to match(/child-src/)
+        expect(firefox_transitional).not_to match(/frame-src/)
+      end
+
       context "browser sniffing" do
         let (:complex_opts) do
           (ContentSecurityPolicy::ALL_DIRECTIVES - [:frame_src]).each_with_object({}) do |directive, hash|