croak.diff

diff --git a/lib/Mojo/UserAgent.pm b/lib/Mojo/UserAgent.pm
index ebe9537..692d2f2 100644
--- a/lib/Mojo/UserAgent.pm
+++ b/lib/Mojo/UserAgent.pm
@@ -3,6 +3,7 @@ use Mojo::Base 'Mojo::EventEmitter';
 
 # "Fry: Since when is the Internet about robbing people of their privacy?
 #  Bender: August 6, 1991."
+use Carp 'croak';
 use Mojo::IOLoop;
 use Mojo::Util qw(monkey_patch term_escape);
 use Mojo::UserAgent::CookieJar;
@@ -48,6 +49,10 @@ sub start {
   # Fork-safety
   $self->_cleanup->server->restart unless ($self->{pid} //= $$) eq $$;
 
+  my $proto = $tx->req->url->protocol;
+  croak qq/Unsupported protocol "$proto"/
+    if $proto && $proto ne 'http' && $proto ne 'https';
+
   # Non-blocking
   if ($cb) {
     warn "-- Non-blocking request (@{[_url($tx)]})\n" if DEBUG;
diff --git a/t/mojo/user_agent.t b/t/mojo/user_agent.t
index de5e604..99ce618 100644
--- a/t/mojo/user_agent.t
+++ b/t/mojo/user_agent.t
@@ -592,4 +592,8 @@ Mojo::IOLoop->delay(
 )->wait;
 is scalar @$result, 2, 'two active connections';
 
+# Unsupported protocol
+eval { $ua->get('file:///etc/passwd') };
+like $@, qr/Unsupported protocol "file"/, 'right error';
+
 done_testing();