add test: invalid table at the top level config

tests/testsuite/config.rs

@@ -128,6 +128,31 @@ fn bad_config_top_level() {
 unknown field `foo`, expected one of `book`, `build`, `rust`, `output`, `preprocessor`
 
 
+"#]]);
+        });
+}
+
+// An invalid table at the top level.
+#[test]
+fn bad_config_top_level_table() {
+    BookTest::init(|_| {})
+        .change_file(
+            "book.toml",
+            "[other]\n\
+            foo = 123",
+        )
+        .run("build", |cmd| {
+            cmd.expect_failure()
+                .expect_stdout(str![[""]])
+                .expect_stderr(str![[r#"
+[TIMESTAMP] [ERROR] (mdbook_core::utils): Error: Invalid configuration file
+[TIMESTAMP] [ERROR] (mdbook_core::utils): [TAB]Caused By: TOML parse error at line 1, column 2
+  |
+1 | [other]
+  |  ^^^^^
+unknown field `other`, expected one of `book`, `build`, `rust`, `output`, `preprocessor`
+
+
 "#]]);
         });
 }