diff --git a/src/protect/engine/request.js b/src/protect/engine/request.js
index 604578b..c0d8055 100644
--- a/src/protect/engine/request.js
+++ b/src/protect/engine/request.js
@@ -253,7 +253,14 @@ export class RequestResolver {
       default: {
         if (key.startsWith('HTTP_')) {
           const headerName = key.substring(5).toLowerCase().replace(/_/g, '-');
-          return req.headers?.[headerName] ? [req.headers[headerName]] : [];
+          const headers = req.headers;
+          // Presence, not truthiness. A header sent with an empty value IS present, and an `isset`
+          // rule authored against it must see it — some bypasses are carried by the header existing
+          // at all, so treating `Header:` as absent would make the rule quietly miss the shape it
+          // was written for. (The named cases above keep value semantics: for host/origin/referer an
+          // empty string and an absent header mean the same thing to the matchers that read them.)
+          if (headers === null || typeof headers !== 'object') return [];
+          return Object.prototype.hasOwnProperty.call(headers, headerName) ? [headers[headerName]] : [];
         }
         return [];
       }
diff --git a/tests/protect/request.test.ts b/tests/protect/request.test.ts
index 03c343a..9d27a07 100644
--- a/tests/protect/request.test.ts
+++ b/tests/protect/request.test.ts
@@ -137,6 +137,26 @@ describe('RequestResolver', () => {
       assert.deepStrictEqual(resolver.resolve('server.HTTP_X_FORWARDED_FOR'), ['1.2.3.4']);
     });
 
+    it('should treat an empty header value as present, not absent', () => {
+      // A header sent with no value IS present, and some bypasses are carried by a header existing at
+      // all rather than by what it contains — so an `isset` rule authored against one must see it.
+      // Resolving by truthiness returned [] here, making the rule silently miss that shape.
+      const resolver = new RequestResolver(createReq({ headers: { 'x-internal-marker': '' } }));
+      assert.deepStrictEqual(resolver.resolve('server.HTTP_X_INTERNAL_MARKER'), ['']);
+    });
+
+    it('should still resolve an absent header to nothing', () => {
+      const resolver = new RequestResolver(createReq({ headers: {} }));
+      assert.deepStrictEqual(resolver.resolve('server.HTTP_X_INTERNAL_MARKER'), []);
+    });
+
+    it('should not treat an inherited property name as a header', () => {
+      // `headers.constructor` exists on every object; reading presence off the prototype chain would
+      // make every request appear to carry a `constructor` header.
+      const resolver = new RequestResolver(createReq({ headers: {} }));
+      assert.deepStrictEqual(resolver.resolve('server.HTTP_CONSTRUCTOR'), []);
+    });
+
     it('should resolve REMOTE_ADDR from ip', () => {
       const resolver = new RequestResolver(createReq({ ip: '10.0.0.1' }));
       assert.deepStrictEqual(resolver.resolve('server.REMOTE_ADDR'), ['10.0.0.1']);