diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index 602465918b0..e52aba1ec7e 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/src/cargo/util/toml/mod.rs
@@ -201,6 +201,12 @@ impl<'de> de::Deserialize<'de> for TomlDependency {
 pub struct DetailedTomlDependency {
     version: Option<String>,
     registry: Option<String>,
+    /// The URL of the `registry` field.
+    /// This is an internal implementation detail. When Cargo creates a
+    /// package, it replaces `registry` with `registry-index` so that the
+    /// manifest contains the correct URL. All users won't have the same
+    /// registry names configured, so Cargo can't rely on just the name for
+    /// crates published by other users.
     registry_index: Option<String>,
     path: Option<String>,
     git: Option<String>,
@@ -1408,6 +1414,12 @@ impl DetailedTomlDependency {
             let registry_id = SourceId::alt_registry(cx.config, registry)?;
             dep.set_registry_id(registry_id);
         }
+        if let Some(registry_index) = &self.registry_index {
+            cx.features.require(Feature::alternative_registries())?;
+            let url = registry_index.to_url()?;
+            let registry_id = SourceId::for_registry(&url)?;
+            dep.set_registry_id(registry_id);
+        }
 
         if let Some(kind) = kind {
             dep.set_kind(kind);
diff --git a/tests/testsuite/alt_registry.rs b/tests/testsuite/alt_registry.rs
index 41ac080b640..f99ab93adc6 100644
--- a/tests/testsuite/alt_registry.rs
+++ b/tests/testsuite/alt_registry.rs
@@ -1063,3 +1063,155 @@ fn alt_reg_metadata() {
         )
         .run();
 }
+
+#[test]
+fn unknown_registry() {
+    // A known registry refers to an unknown registry.
+    // foo -> bar(crates.io) -> baz(alt)
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            cargo-features = ["alternative-registries"]
+
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+
+            [dependencies.bar]
+            version = "0.0.1"
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    Package::new("baz", "0.0.1")
+        .alternative(true)
+        .publish();
+    Package::new("bar", "0.0.1")
+        .registry_dep("baz", "0.0.1")
+        .publish();
+
+    // Remove "alternative" from config.
+    let cfg_path = paths::home().join(".cargo/config");
+    let mut config = fs::read_to_string(&cfg_path).unwrap();
+    let start = config.find("[registries.alternative]").unwrap();
+    config.insert(start, '#');
+    let start_index = &config[start..].find("index =").unwrap();
+    config.insert(start + start_index, '#');
+    fs::write(&cfg_path, config).unwrap();
+
+    p.cargo("build")
+        .masquerade_as_nightly_cargo()
+        .run();
+
+    // Important parts:
+    // foo -> bar registry = null
+    // bar -> baz registry = alternate
+    p.cargo("metadata --format-version=1")
+        .masquerade_as_nightly_cargo()
+        .with_json(r#"
+            {
+              "packages": [
+                {
+                  "name": "baz",
+                  "version": "0.0.1",
+                  "id": "baz 0.0.1 (registry+file://[..]/alternative-registry)",
+                  "license": null,
+                  "license_file": null,
+                  "description": null,
+                  "source": "registry+file://[..]/alternative-registry",
+                  "dependencies": [],
+                  "targets": "{...}",
+                  "features": {},
+                  "manifest_path": "[..]",
+                  "metadata": null,
+                  "authors": [],
+                  "categories": [],
+                  "keywords": [],
+                  "readme": null,
+                  "repository": null,
+                  "edition": "2015",
+                  "links": null
+                },
+                {
+                  "name": "foo",
+                  "version": "0.0.1",
+                  "id": "foo 0.0.1 (path+file://[..]/foo)",
+                  "license": null,
+                  "license_file": null,
+                  "description": null,
+                  "source": null,
+                  "dependencies": [
+                    {
+                      "name": "bar",
+                      "source": "registry+https://github.com/rust-lang/crates.io-index",
+                      "req": "^0.0.1",
+                      "kind": null,
+                      "rename": null,
+                      "optional": false,
+                      "uses_default_features": true,
+                      "features": [],
+                      "target": null,
+                      "registry": null
+                    }
+                  ],
+                  "targets": "{...}",
+                  "features": {},
+                  "manifest_path": "[..]/foo/Cargo.toml",
+                  "metadata": null,
+                  "authors": [],
+                  "categories": [],
+                  "keywords": [],
+                  "readme": null,
+                  "repository": null,
+                  "edition": "2015",
+                  "links": null
+                },
+                {
+                  "name": "bar",
+                  "version": "0.0.1",
+                  "id": "bar 0.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
+                  "license": null,
+                  "license_file": null,
+                  "description": null,
+                  "source": "registry+https://github.com/rust-lang/crates.io-index",
+                  "dependencies": [
+                    {
+                      "name": "baz",
+                      "source": "registry+file://[..]/alternative-registry",
+                      "req": "^0.0.1",
+                      "kind": null,
+                      "rename": null,
+                      "optional": false,
+                      "uses_default_features": true,
+                      "features": [],
+                      "target": null,
+                      "registry": "file:[..]/alternative-registry"
+                    }
+                  ],
+                  "targets": "{...}",
+                  "features": {},
+                  "manifest_path": "[..]",
+                  "metadata": null,
+                  "authors": [],
+                  "categories": [],
+                  "keywords": [],
+                  "readme": null,
+                  "repository": null,
+                  "edition": "2015",
+                  "links": null
+                }
+              ],
+              "workspace_members": [
+                "foo 0.0.1 (path+file://[..]/foo)"
+              ],
+              "resolve": "{...}",
+              "target_directory": "[..]/foo/target",
+              "version": 1,
+              "workspace_root": "[..]/foo"
+            }
+            "#)
+        .run();
+}
diff --git a/tests/testsuite/support/registry.rs b/tests/testsuite/support/registry.rs
index 82439c15c91..76b641c5384 100644
--- a/tests/testsuite/support/registry.rs
+++ b/tests/testsuite/support/registry.rs
@@ -454,7 +454,8 @@ impl Package {
                 target, kind, dep.name, dep.vers
             ));
             if let Some(registry) = &dep.registry {
-                manifest.push_str(&format!("registry = \"{}\"", registry));
+                assert_eq!(registry, "alternative");
+                manifest.push_str(&format!("registry-index = \"{}\"", alt_registry()));
             }
         }