diff --git a/Gemfile.lock b/Gemfile.lock
index 563692f8e0..43123dfc51 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -772,7 +772,7 @@ GEM
     webfinger (1.2.0)
       activesupport
       httpclient (>= 2.4)
-    webmock (3.18.1)
+    webmock (3.19.1)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)
       hashdiff (>= 0.4.0, < 2.0.0)
diff --git a/spec/system/admin/invoice_print_spec.rb b/spec/system/admin/invoice_print_spec.rb
index 790db07849..45a6823706 100644
--- a/spec/system/admin/invoice_print_spec.rb
+++ b/spec/system/admin/invoice_print_spec.rb
@@ -29,7 +29,12 @@ describe '
 
   before do
     Capybara.current_driver = :rack_test
-    stub_request(:get, ->(uri) { uri.to_s.include? "/css/mail" })
+
+    # return a duplicate empaty string for CSS pack request like:
+    # 'http://test.host/packs-test/css/mail-1ab2dc7f.css'
+    # This is because Wicked PDF will try to force an encoding on the returned string, which will
+    # break with a frozen string
+    stub_request(:get, ->(uri) { uri.to_s.include? "/css/mail" }).to_return(body: "".dup)
   end
 
   after do
diff --git a/spec/views/spree/admin/orders/invoice.html.haml_spec.rb b/spec/views/spree/admin/orders/invoice.html.haml_spec.rb
index e6cccbdced..f0aebd480e 100644
--- a/spec/views/spree/admin/orders/invoice.html.haml_spec.rb
+++ b/spec/views/spree/admin/orders/invoice.html.haml_spec.rb
@@ -27,7 +27,11 @@ describe "spree/admin/orders/invoice.html.haml" do
                                     display_checkout_total_less_tax: '8',
                                     outstanding_balance_label: 'Outstanding Balance'
 
-    stub_request(:get, ->(uri) { uri.to_s.include? "/css/mail" })
+    # return a duplicate empaty string for CSS pack request like:
+    # 'http://test.host/packs-test/css/mail-1ab2dc7f.css'
+    # This is because Wicked PDF will try to force an encoding on the returned string, which will
+    # break with a frozen string
+    stub_request(:get, ->(uri) { uri.to_s.include? "/css/mail" }).to_return(body: "".dup)
   end
 
   it "displays the customer code" do