diff --git a/index.js b/index.js
index 379f2a98..3fdac9a4 100644
--- a/index.js
+++ b/index.js
@@ -901,7 +901,17 @@ function buildValue (location, input) {
           switch (type) {
             case 'string': {
               code += `
-                ${statement}(${input} === null || typeof ${input} === "${type}" || ${input} instanceof RegExp || (typeof ${input} === "object" && Object.prototype.hasOwnProperty.call(${input}, "toString")))
+                ${statement}(
+                  typeof ${input} === "string" ||
+                  ${input} === null ||
+                  ${input} instanceof RegExp ||
+                  (
+                    typeof ${input} === "object" &&
+                    typeof ${input}.toString === "function" &&
+                    ${input}.toString !== Object.prototype.toString &&
+                    !(${input} instanceof Date)
+                  )
+                )
                   ${nestedResult}
               `
               break
diff --git a/test/typesArray.test.js b/test/typesArray.test.js
index fa840fdf..c6631a03 100644
--- a/test/typesArray.test.js
+++ b/test/typesArray.test.js
@@ -438,6 +438,35 @@ test('object that is simultaneously a string and a json switched', (t) => {
   t.equal(valueObj, '{"simultaneously":{"foo":"hello"}}')
 })
 
+test('class instance that is simultaneously a string and a json', (t) => {
+  t.plan(2)
+
+  const schema = {
+    type: 'object',
+    properties: {
+      simultaneously: {
+        type: ['string', 'object'],
+        properties: {
+          foo: { type: 'string' }
+        }
+      }
+    }
+  }
+
+  class Test {
+    toString () { return 'hello' }
+  }
+
+  const likeObjectId = new Test()
+
+  const stringify = build(schema)
+  const valueStr = stringify({ simultaneously: likeObjectId })
+  t.equal(valueStr, '{"simultaneously":"hello"}')
+
+  const valueObj = stringify({ simultaneously: { foo: likeObjectId } })
+  t.equal(valueObj, '{"simultaneously":{"foo":"hello"}}')
+})
+
 test('should throw an error when type is array and object is null', (t) => {
   t.plan(1)
   const schema = {