diff --git a/src/cargo/util/config.rs b/src/cargo/util/config.rs
index b3075a99a41..4494b9ca307 100644
--- a/src/cargo/util/config.rs
+++ b/src/cargo/util/config.rs
@@ -29,7 +29,7 @@ use crate::util::errors::{self, internal, CargoResult, CargoResultExt};
 use crate::util::toml as cargo_toml;
 use crate::util::Filesystem;
 use crate::util::Rustc;
-use crate::util::ToUrlWithBase;
+use crate::util::{ToUrl, ToUrlWithBase};
 use crate::util::{paths, validate_package_name};
 
 /// Configuration information for cargo. This is not specific to a build, it is information
@@ -684,7 +684,9 @@ impl Config {
     }
 
     fn resolve_registry_index(&self, index: Value<String>) -> CargoResult<Url> {
-        let base = index.definition.root(&self).join("truncated-by-url-with-base");
+        let base = index.definition.root(&self).join("truncated-by-url_with_base");
+        // Parse val to check it is a URL, not a relative path without a protocol.
+        let _parsed = index.val.to_url()?;
         let url = index.val.to_url_with_base(Some(&*base))?;
         if url.password().is_some() {
             failure::bail!("Registry URLs may not contain passwords");
diff --git a/tests/testsuite/alt_registry.rs b/tests/testsuite/alt_registry.rs
index d4b1d3bc1a9..0f5491836ef 100644
--- a/tests/testsuite/alt_registry.rs
+++ b/tests/testsuite/alt_registry.rs
@@ -1167,13 +1167,13 @@ fn unknown_registry() {
 }
 
 #[test]
-fn registries_index_relative_path() {
+fn registries_index_relative_url() {
     let config = paths::root().join(".cargo/config");
     fs::create_dir_all(config.parent().unwrap()).unwrap();
     File::create(&config).unwrap()
         .write_all(br#"
             [registries.relative]
-            index = "alternative-registry"
+            index = "file:alternative-registry"
         "#).unwrap();
 
     registry::init();
@@ -1215,13 +1215,13 @@ fn registries_index_relative_path() {
 }
 
 #[test]
-fn registry_index_relative_path() {
+fn registry_index_relative_url() {
     let config = paths::root().join(".cargo/config");
     fs::create_dir_all(config.parent().unwrap()).unwrap();
     File::create(&config).unwrap()
         .write_all(br#"
             [registry]
-            index = "alternative-registry"
+            index = "file:alternative-registry"
         "#).unwrap();
 
     registry::init();
@@ -1263,3 +1263,49 @@ warning: custom registry support via the `registry.index` configuration is being
         ))
         .run();
 }
+
+#[test]
+fn registries_index_relative_path_not_allowed() {
+    let config = paths::root().join(".cargo/config");
+    fs::create_dir_all(config.parent().unwrap()).unwrap();
+    File::create(&config).unwrap()
+        .write_all(br#"
+            [registries.relative]
+            index = "alternative-registry"
+        "#).unwrap();
+
+    registry::init();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+
+            [dependencies.bar]
+            version = "0.0.1"
+            registry = "relative"
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    Package::new("bar", "0.0.1")
+        .alternative(true)
+        .publish();
+
+    p.cargo("build")
+        .with_stderr(&format!(
+            "\
+error: failed to parse manifest at `{root}/foo/Cargo.toml`
+
+Caused by:
+  invalid url `alternative-registry`: relative URL without a base
+"
+        , root = paths::root().to_str().unwrap()))
+        .with_status(101)
+        .run();
+}