diff --git a/lib/http/request.rb b/lib/http/request.rb
index 1372cd11..f576d290 100644
--- a/lib/http/request.rb
+++ b/lib/http/request.rb
@@ -169,11 +169,13 @@ def connect_using_proxy(socket)
     def headline
       request_uri =
         if using_proxy? && !uri.https?
-          uri.omit(:fragment)
+          uri.omit(:fragment).to_s
         else
-          uri.request_uri
+          uri.request_uri.to_s
         end
 
+      raise RequestError, "Invalid request URI: #{request_uri.inspect}" if request_uri.match?(/\s/)
+
       "#{verb.to_s.upcase} #{request_uri} HTTP/#{version}"
     end
 
@@ -230,7 +232,11 @@ def port
 
     # @return [String] Default host (with port if needed) header value.
     def default_host_header_value
-      PORTS[@scheme] == port ? host : "#{host}:#{port}"
+      value = PORTS[@scheme] == port ? host : "#{host}:#{port}"
+
+      raise RequestError, "Invalid host: #{value.inspect}" if value.match?(/\s/)
+
+      value
     end
 
     def prepare_body(body)
diff --git a/spec/lib/http_spec.rb b/spec/lib/http_spec.rb
index f7a1bb21..746a08ec 100644
--- a/spec/lib/http_spec.rb
+++ b/spec/lib/http_spec.rb
@@ -460,20 +460,37 @@ def setsockopt(*args)
 
     context "with :normalize_uri" do
       it "normalizes URI" do
-        response = HTTP.get "#{dummy.endpoint}/hello world"
+        response = HTTP.get "#{dummy.endpoint}/héllö-wörld"
         expect(response.to_s).to eq("hello world")
       end
 
       it "uses the custom URI Normalizer method" do
         client = HTTP.use(:normalize_uri => {:normalizer => :itself.to_proc})
-        response = client.get("#{dummy.endpoint}/hello world")
+        response = client.get("#{dummy.endpoint}/héllö-wörld")
         expect(response.status).to eq(400)
       end
 
+      it "raises if custom URI Normalizer returns invalid path" do
+        client = HTTP.use(:normalize_uri => {:normalizer => :itself.to_proc})
+        expect { client.get("#{dummy.endpoint}/hello\nworld") }.
+          to raise_error HTTP::RequestError, 'Invalid request URI: "/hello\nworld"'
+      end
+
+      it "raises if custom URI Normalizer returns invalid host" do
+        normalizer = lambda do |uri|
+          uri.port = nil
+          uri.instance_variable_set(:@host, "example\ncom")
+          uri
+        end
+        client = HTTP.use(:normalize_uri => {:normalizer => normalizer})
+        expect { client.get(dummy.endpoint) }.
+          to raise_error HTTP::RequestError, 'Invalid host: "example\ncom"'
+      end
+
       it "uses the default URI normalizer" do
         client = HTTP.use :normalize_uri
         expect(HTTP::URI::NORMALIZER).to receive(:call).and_call_original
-        response = client.get("#{dummy.endpoint}/hello world")
+        response = client.get("#{dummy.endpoint}/héllö-wörld")
         expect(response.to_s).to eq("hello world")
       end
     end
diff --git a/spec/support/dummy_server/servlet.rb b/spec/support/dummy_server/servlet.rb
index 9f835c33..81e32ab4 100644
--- a/spec/support/dummy_server/servlet.rb
+++ b/spec/support/dummy_server/servlet.rb
@@ -149,7 +149,7 @@ def do_#{method.upcase}(req, res)
       res.body   = req.body
     end
 
-    get "/hello world" do |_req, res|
+    get "/héllö-wörld".b do |_req, res|
       res.status = 200
       res.body   = "hello world"
     end