diff --git a/src/Flurl.Http/FlurlHttp.cs b/src/Flurl.Http/FlurlHttp.cs
index 7c633c5d..bfd69cba 100644
--- a/src/Flurl.Http/FlurlHttp.cs
+++ b/src/Flurl.Http/FlurlHttp.cs
@@ -48,6 +48,6 @@ public static IFlurlClientBuilder ConfigureClientForUrl(string url) {
 		/// <summary>
 		/// Builds a cache key consisting of URL scheme, host, and port. This is the default client caching strategy.
 		/// </summary>
-		public static string BuildClientNameByHost(IFlurlRequest req) => $"{req.Url.Scheme}|{req.Url.Host}|{req.Url.Port}";
+		public static string BuildClientNameByHost(IFlurlRequest req) => $"{req.Url?.Scheme}|{req.Url?.Host}|{req.Url?.Port}";
 	}
 }
\ No newline at end of file
diff --git a/src/Flurl.Http/FlurlRequest.cs b/src/Flurl.Http/FlurlRequest.cs
index ca9762b4..c98e6649 100644
--- a/src/Flurl.Http/FlurlRequest.cs
+++ b/src/Flurl.Http/FlurlRequest.cs
@@ -115,6 +115,7 @@ public IFlurlClient Client {
 			set {
 				_client = value;
 				Settings.Parent = _client?.Settings;
+				SyncBaseUrl(_client, this);
 				SyncHeaders(_client, this);
 			}
 		}
@@ -163,6 +164,19 @@ internal static void SyncHeaders(IFlurlClient client, IFlurlRequest request) {
 			}
 		}
 
+		/// <summary>
+		/// Prepends client.BaseUrl to this.Url, but only if this.Url isn't already a valid, absolute URL.
+		/// </summary>
+		private static void SyncBaseUrl(IFlurlClient client, IFlurlRequest request) {
+			if (string.IsNullOrEmpty(client?.BaseUrl))
+				return;
+
+			if (request.Url == null)
+				request.Url = client.BaseUrl;
+			else if (!Url.IsValid(request.Url))
+				request.Url = Url.Combine(client.BaseUrl, request.Url);
+		}
+
 		private void ApplyCookieJar(CookieJar jar) {
 			_jar = jar;
 			if (jar == null)
diff --git a/test/Flurl.Test/Http/GlobalConfigTests.cs b/test/Flurl.Test/Http/GlobalConfigTests.cs
index fecd033b..73ffb61d 100644
--- a/test/Flurl.Test/Http/GlobalConfigTests.cs
+++ b/test/Flurl.Test/Http/GlobalConfigTests.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Threading.Tasks;
 using Flurl.Http;
+using Flurl.Http.Testing;
 using NUnit.Framework;
 
 namespace Flurl.Test.Http
@@ -74,5 +75,23 @@ public void can_configure_client_for_url() {
 			Assert.AreEqual(123, cli2.Settings.Timeout.Value.TotalSeconds);
 			Assert.AreNotEqual(123, cli3.Settings.Timeout.Value.TotalSeconds);
 		}
+
+		[Test] // #803
+		public async Task can_prepend_global_base_url() {
+			FlurlHttp.Clients.WithDefaults(builder =>
+				builder.ConfigureHttpClient(cli => cli.BaseAddress = new Uri("https://api.com")));
+
+			using var test = new HttpTest();
+
+			await "".GetAsync();
+			await "/path/1".GetAsync();
+			await "path/2".GetAsync();
+			await "https://other.com/path/3".GetAsync();
+
+			test.ShouldHaveCalled("https://api.com/").Times(1);
+			test.ShouldHaveCalled("https://api.com/path/1").Times(1);
+			test.ShouldHaveCalled("https://api.com/path/2").Times(1);
+			test.ShouldHaveCalled("https://other.com/path/3").Times(1);
+		}
 	}
 }