diff --git a/src/args.ts b/src/args.ts
index 3642a12..3e37952 100644
--- a/src/args.ts
+++ b/src/args.ts
@@ -131,7 +131,11 @@ export function parseFlags(argv: string[], options: OptionDef[]): GlobalFlags {
         if (arr) arr.push(value);
         else (flags as Record<string, unknown>)[camelKey] = [value];
       } else if (schema.numbers.has(camelKey)) {
-        (flags as Record<string, unknown>)[camelKey] = Number(value);
+        const numericValue = Number(value);
+        if (value.trim() === '' || !Number.isFinite(numericValue)) {
+          throw new Error(`Flag --${key} requires a numeric value, got "${value}".`);
+        }
+        (flags as Record<string, unknown>)[camelKey] = numericValue;
       } else {
         (flags as Record<string, unknown>)[camelKey] = value;
       }
diff --git a/test/args.test.ts b/test/args.test.ts
new file mode 100644
index 0000000..e71ac2c
--- /dev/null
+++ b/test/args.test.ts
@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'bun:test';
+import { parseFlags } from '../src/args';
+import type { OptionDef } from '../src/command';
+
+const OPTIONS: OptionDef[] = [
+  { flag: '--timeout <seconds>', description: 'Request timeout', type: 'number' },
+  { flag: '--message <text>', description: 'Message text', type: 'array' },
+];
+
+describe('parseFlags', () => {
+  it('rejects non-numeric values for number flags', () => {
+    expect(() => parseFlags(['--timeout', 'abc'], OPTIONS)).toThrow(
+      'Flag --timeout requires a numeric value, got "abc".',
+    );
+  });
+
+  it('rejects empty values for number flags', () => {
+    expect(() => parseFlags(['--timeout='], OPTIONS)).toThrow(
+      'Flag --timeout requires a numeric value, got "".',
+    );
+  });
+
+  it('still accepts finite numeric values', () => {
+    const flags = parseFlags(['--timeout', '1.5'], OPTIONS);
+
+    expect(flags.timeout).toBe(1.5);
+  });
+});