diff --git a/lib/test.js b/lib/test.js
index 3b3b255..27fb76c 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -308,6 +308,12 @@ function wrapAssertFn(assertFn) {
       err = assertFn(res);
     } catch (e) {
       err = e;
+
+      // Handle errors that fail `instanceof Error`, like `throw 1;` or dealing with multiple realms/contexts:
+      // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof#instanceof_and_multiple_realms
+      if (!(err instanceof Error)) {
+        err = new Error(err);
+      }
     }
     if (err instanceof Error && err.stack) {
       badStack = err.stack.replace(err.message, '').split('\n').slice(1);
diff --git a/test/supertest.js b/test/supertest.js
index 9eebf31..c36b665 100644
--- a/test/supertest.js
+++ b/test/supertest.js
@@ -871,6 +871,20 @@ describe('request(app)', function () {
           .expect('Content-Type', /text/)
           .end(done);
       });
+
+      it('fails for non-Error values that are thrown', function (done) {
+        get
+          .expect(function (res) {
+            // eslint-disable-next-line no-throw-literal
+            throw 'error123';
+          })
+          .end(function (err) {
+            should.exist(err);
+            err.message.should.equal('error123');
+            shouldIncludeStackWithThisFile(err);
+            done();
+          });
+      });
     });
 
     describe('handling multiple assertions per field', function () {