diff --git a/src/bin/rustup-init.rs b/src/bin/rustup-init.rs
index 09b125b87c..07820eeb9a 100644
--- a/src/bin/rustup-init.rs
+++ b/src/bin/rustup-init.rs
@@ -29,6 +29,7 @@ use rustup::cli::setup_mode;
 use rustup::currentprocess::{process, with, OSProcess};
 use rustup::env_var::RUST_RECURSION_COUNT_MAX;
 use rustup::utils::utils;
+use rustup::TOOLS;
 
 fn main() {
     let process = OSProcess::default();
@@ -93,7 +94,13 @@ fn run_rustup_inner() -> Result<utils::ExitCode> {
                 }
             }
         }
-        Some(_) => proxy_mode::main(),
+        Some(n) => {
+            if TOOLS.iter().find(|&&name| name == n).is_some() {
+                proxy_mode::main()
+            } else {
+                Err(ErrorKind::UnknownProxyName(n.to_string()).into())
+            }
+        }
         None => {
             // Weird case. No arg0, or it's unparsable.
             Err(ErrorKind::NoExeName.into())
diff --git a/src/cli/errors.rs b/src/cli/errors.rs
index 7b5a6ba849..c6f413c55b 100644
--- a/src/cli/errors.rs
+++ b/src/cli/errors.rs
@@ -13,6 +13,7 @@ use strsim::damerau_levenshtein;
 
 use super::rustup_mode::CompletionCommand;
 use crate::dist::temp;
+use crate::TOOLS;
 
 error_chain! {
     links {
@@ -47,6 +48,14 @@ error_chain! {
         NoExeName {
             description("couldn't determine self executable name")
         }
+        UnknownProxyName(n: String) {
+            description("unknown proxy name")
+            display(
+                "unknown proxy name: '{}'; valid proxy names are {}",
+                n,
+                valid_proxy_names(),
+            )
+        }
         NotSelfInstalled(p: PathBuf) {
             description("rustup is not installed")
             display("rustup is not installed at '{}'", p.display())
@@ -74,6 +83,14 @@ error_chain! {
     }
 }
 
+fn valid_proxy_names() -> String {
+    TOOLS
+        .iter()
+        .map(|s| format!("'{}'", s))
+        .collect::<Vec<_>>()
+        .join(", ")
+}
+
 fn maybe_suggest_toolchain(bad_name: &str) -> String {
     let bad_name = &bad_name.to_ascii_lowercase();
     static VALID_CHANNELS: &[&str] = &["stable", "beta", "nightly"];
diff --git a/tests/cli-misc.rs b/tests/cli-misc.rs
index 92b7619d8a..20f1287902 100644
--- a/tests/cli-misc.rs
+++ b/tests/cli-misc.rs
@@ -325,10 +325,7 @@ fn rustup_failed_path_search() {
         expect_err(
             config,
             broken,
-            &format!(
-                "'fake_proxy{}' is not installed for the toolchain 'custom'",
-                EXE_SUFFIX
-            ),
+            &format!("unknown proxy name: 'fake_proxy'; valid proxy names are 'rustc', 'rustdoc', 'cargo', 'rust-lldb', 'rust-gdb', 'rls', 'cargo-clippy', 'clippy-driver', 'cargo-miri'",),
         );
 
         // Hardlink will be automatically cleaned up by test setup code