diff --git a/.changeset/loud-pandas-repeat.md b/.changeset/loud-pandas-repeat.md
new file mode 100644
index 0000000..28dea61
--- /dev/null
+++ b/.changeset/loud-pandas-repeat.md
@@ -0,0 +1,9 @@
+---
+"@stll/stdnum": patch
+---
+
+Say which package is missing when the native binding cannot load. The platform
+binaries are optional dependencies, so an installer that declines them leaves
+the package importable and dead, throwing only on first use. The previous error
+listed every supported target, including the one it was running on, which reads
+as a portability problem rather than a missing install.
diff --git a/packages/stdnum/src/__test__/native-node.test.ts b/packages/stdnum/src/__test__/native-node.test.ts
new file mode 100644
index 0000000..146a0fd
--- /dev/null
+++ b/packages/stdnum/src/__test__/native-node.test.ts
@@ -0,0 +1,110 @@
+/**
+ * The loader had no tests, which is how a real failure stayed invisible: the
+ * platform binaries are optionalDependencies, so an installer that declines
+ * them leaves this package importable and dead, throwing only on first use.
+ * The message that surfaced then said "supported targets: …, linux-x64-gnu,
+ * …" while failing ON linux-x64-gnu, which points the reader at a
+ * portability problem that does not exist.
+ *
+ * Every input the loader consults is injectable, so both branches are
+ * testable without touching the filesystem or the real platform.
+ */
+
+import { describe, expect, test } from "bun:test";
+
+import type { NativeStdnumBinding } from "../native";
+import { loadNativeStdnumBinding } from "../native-node";
+
+/** Nothing resolves: the shape an install with no platform package has. */
+const requireNothing = (specifier: string): unknown => {
+  throw new Error(`Cannot find module '${specifier}'`);
+};
+
+describe("loadNativeStdnumBinding", () => {
+  test("names the uninstalled package when the platform IS a build target", () => {
+    expect(() =>
+      loadNativeStdnumBinding({
+        platform: "linux",
+        arch: "x64",
+        libc: "gnu",
+        env: {},
+        requireModule: requireNothing,
+      }),
+    ).toThrow(/@stll\/stdnum-linux-x64-gnu/u);
+  });
+
+  test("says the target exists rather than blaming the platform", () => {
+    let message = "";
+    try {
+      loadNativeStdnumBinding({
+        platform: "darwin",
+        arch: "arm64",
+        env: {},
+        requireModule: requireNothing,
+      });
+    } catch (error) {
+      message =
+        error instanceof Error
+          ? error.message
+          : String(error);
+    }
+
+    expect(message).toContain("IS a build target");
+    expect(message).toContain("optionalDependency");
+    // The old message listed every supported target and nothing else, which
+    // read as "your platform is unsupported" for a platform that is.
+    expect(message).not.toMatch(
+      /^Unable to load native stdnum binding.*supported targets/u,
+    );
+  });
+
+  test("reports an unsupported platform as unsupported", () => {
+    let message = "";
+    try {
+      loadNativeStdnumBinding({
+        platform: "linux",
+        arch: "x64",
+        libc: "musl",
+        env: {},
+        requireModule: requireNothing,
+      });
+    } catch (error) {
+      message =
+        error instanceof Error
+          ? error.message
+          : String(error);
+    }
+
+    expect(message).toContain(
+      "not among the build targets",
+    );
+    expect(message).toContain("linux-x64-musl");
+  });
+
+  test("prefers an explicit library path over the platform package", () => {
+    // Every member the binding contract requires; a partial object is
+    // rejected as "does not match the stdnum binding contract".
+    const binding = new Proxy(
+      {},
+      { get: () => () => undefined },
+    ) as NativeStdnumBinding;
+    const loaded = loadNativeStdnumBinding({
+      platform: "linux",
+      arch: "x64",
+      libc: "gnu",
+      env: {
+        STELLA_STDNUM_NATIVE_LIBRARY_PATH:
+          "/tmp/custom.node",
+      },
+      requireModule: (specifier) => {
+        if (specifier === "/tmp/custom.node")
+          return binding;
+        throw new Error(
+          `Cannot find module '${specifier}'`,
+        );
+      },
+    });
+
+    expect(loaded).toBe(binding);
+  });
+});
diff --git a/packages/stdnum/src/native-node.ts b/packages/stdnum/src/native-node.ts
index 77792d8..122c4de 100644
--- a/packages/stdnum/src/native-node.ts
+++ b/packages/stdnum/src/native-node.ts
@@ -69,12 +69,31 @@ export const loadNativeStdnumBinding = (
       );
     }
   }
+  const current = [platform, arch, libc]
+    .filter(Boolean)
+    .join("-");
   const supported = targets
     .map(([p, a, l]) => [p, a, l].filter(Boolean).join("-"))
     .join(", ");
+
+  // Distinguish "we do not build for this platform" from "we do, but the
+  // package holding the binary is not installed". They read identically in a
+  // stack trace and have completely different fixes, and the second is the
+  // common one: the platform packages are optionalDependencies, so anything
+  // that declines to install them — `--no-optional`, an install-time version
+  // gate such as a release-age policy, a partial lockfile — leaves this
+  // package importable and non-functional, with no error until first use.
+  const reason =
+    match === undefined
+      ? `This platform is not among the build targets (${supported}).`
+      : `This platform IS a build target, so ${match[3]} exists but was not ` +
+        `installed. It is an optionalDependency: check that optional ` +
+        `dependencies are enabled, that ${match[3]} is not excluded by an ` +
+        `install policy, and that it appears in your lockfile.`;
+
   throw new Error(
-    `Unable to load native stdnum binding for ${[platform, arch, libc].filter(Boolean).join("-")}; ` +
-      `supported targets: ${supported}.\n${errors.join("\n")}`,
+    `Unable to load the native stdnum binding for ${current}. ${reason}\n` +
+      `Tried:\n${errors.map((line) => `  ${line}`).join("\n")}`,
   );
 };