diff --git a/clap_derive/src/derives/subcommand.rs b/clap_derive/src/derives/subcommand.rs
index 40e6ce45..ba4192b2 100644
--- a/clap_derive/src/derives/subcommand.rs
+++ b/clap_derive/src/derives/subcommand.rs
@@ -188,12 +188,18 @@ fn gen_from_subcommand(
                                 quote!(::std::string::String),
                                 quote!(values_of),
                             )
-                        } else {
+                        } else if is_simple_ty(subty, "OsString") {
                             (
                                 subty.span(),
                                 quote!(::std::ffi::OsString),
                                 quote!(values_of_os),
                             )
+                        } else {
+                            abort!(
+                                ty.span(),
+                                "The type must be either `Vec<String>` or `Vec<OsString>` \
+                                 to be used with `external_subcommand`."
+                            );
                         }
                     }
 
diff --git a/clap_derive/tests/ui/external_subcommand_wrong_type.stderr b/clap_derive/tests/ui/external_subcommand_wrong_type.stderr
index d4ff9026..21d1bb37 100644
--- a/clap_derive/tests/ui/external_subcommand_wrong_type.stderr
+++ b/clap_derive/tests/ui/external_subcommand_wrong_type.stderr
@@ -1,3 +1,9 @@
+error: The type must be either `Vec<String>` or `Vec<OsString>` to be used with `external_subcommand`.
+ --> $DIR/external_subcommand_wrong_type.rs:7:11
+  |
+7 |     Other(Vec<CString>),
+  |           ^^^
+
 error: The type must be either `Vec<String>` or `Vec<OsString>` to be used with `external_subcommand`.
   --> $DIR/external_subcommand_wrong_type.rs:13:11
    |
@@ -10,12 +16,3 @@ error: The enum variant marked with `external_attribute` must be a single-typed
 18 | /     #[clap(external_subcommand)]
 19 | |     Other { a: String },
    | |_______________________^
-
-error[E0308]: mismatched types
- --> $DIR/external_subcommand_wrong_type.rs:7:15
-  |
-7 |     Other(Vec<CString>),
-  |               ^^^^^^^ expected struct `std::ffi::CString`, found struct `std::ffi::OsString`
-  |
-  = note: expected struct `std::vec::Vec<std::ffi::CString>`
-             found struct `std::vec::Vec<std::ffi::OsString>`