diff --git a/crates/nucleus-cli/src/main.rs b/crates/nucleus-cli/src/main.rs
index ae0f945..61a646d 100644
--- a/crates/nucleus-cli/src/main.rs
+++ b/crates/nucleus-cli/src/main.rs
@@ -672,14 +672,19 @@ fn run_lockstep(path: &Path, explain: bool) -> ExitCode {
         }
     };
 
-    let check_report = match nucleus_compiler::check(&text) {
-        Ok(report) => report,
+    let (check_report, family_warning) = match check_family(&text) {
+        Ok(result) => result,
         Err(err) => {
             print_parse_error(&toml_path, &err);
             return ExitCode::FAILURE;
         }
     };
 
+    if let Some(warning) = &family_warning {
+        eprintln!("warning: {warning}");
+        eprintln!("         falling back to STM32F446RE; results may be inaccurate.\n");
+    }
+
     if !check_report.is_ok() {
         let n = check_report.conflicts.len();
         eprintln!(
diff --git a/crates/nucleus-cli/tests/cli.rs b/crates/nucleus-cli/tests/cli.rs
index 0377235..37a82ec 100644
--- a/crates/nucleus-cli/tests/cli.rs
+++ b/crates/nucleus-cli/tests/cli.rs
@@ -712,6 +712,28 @@ fn lockstep_with_present_firmware_runs_both_backends_concurrently_without_hangin
     );
 }
 
+#[test]
+fn lockstep_warns_on_unknown_family_instead_of_silently_falling_back() {
+    // Regression test for issue 53: run_lockstep called check() instead of
+    // check_family(), so an unrecognized [device].family silently validated
+    // against the F446RE fallback DB with no warning — unlike `check` and
+    // `test`, which both surface UnknownFamily via check_family()/test_plan().
+    let proj = TempProject::new();
+    std::fs::write(
+        proj.path().join("stm32.toml"),
+        "[device]\nfamily = \"STM32H750\"\n",
+    )
+    .unwrap();
+
+    let out = nucleus(&["lockstep".as_ref(), proj.path().as_os_str()]);
+
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        stderr.contains("unsupported device family"),
+        "expected an UnknownFamily warning, got stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn init_rejects_unknown_board() {
     let proj = TempProject::new();