diff --git a/src/wrapper.ts b/src/wrapper.ts
index 1f002d4..e15af74 100644
--- a/src/wrapper.ts
+++ b/src/wrapper.ts
@@ -44,7 +44,7 @@ export namespace Signal {
     #brand() {}
 
     static {
-      isState = (s) => #brand in s;
+      isState = (s) => typeof s === 'object' && #brand in s;
     }
 
     constructor(initialValue: T, options: Signal.Options<T> = {}) {
@@ -84,7 +84,7 @@ export namespace Signal {
     #brand() {}
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     static {
-      isComputed = (c: any) => #brand in c;
+      isComputed = (c: any) => typeof c === 'object' && #brand in c;
     }
 
     // Create a Signal which evaluates to the value returned by the callback.
diff --git a/tests/behaviors/guards.test.ts b/tests/behaviors/guards.test.ts
new file mode 100644
index 0000000..d66a143
--- /dev/null
+++ b/tests/behaviors/guards.test.ts
@@ -0,0 +1,22 @@
+import {describe, expect, it} from 'vitest';
+import {Signal} from '../../src/wrapper.js';
+
+describe('Guards', () => {
+  it('should work with Signals', () => {
+    const state = new Signal.State(1);
+    const computed = new Signal.Computed(() => state.get() * 2);
+    expect(Signal.isState(state)).toBe(true);
+    expect(Signal.isComputed(state)).toBe(false);
+
+    expect(Signal.isState(computed)).toBe(false);
+    expect(Signal.isComputed(computed)).toBe(true);
+  });
+
+  it("shouldn't error with values", () => {
+    expect(Signal.isState(1)).toBe(false);
+    expect(Signal.isComputed(2)).toBe(false);
+
+    expect(Signal.isState({})).toBe(false);
+    expect(Signal.isComputed({})).toBe(false);
+  });
+});