diff --git a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
index b48538bfdf083..4168b40cb7afc 100644
--- a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
@@ -2604,7 +2604,7 @@ export function pushStartInstance(
       );
     }
     default: {
-      if (type.indexOf('-') === -1 && typeof props.is !== 'string') {
+      if (type.indexOf('-') === -1) {
         // Generic element
         return pushStartGenericElement(target, props, type);
       } else {
diff --git a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
index 64dc7126ad6d4..e941899ab4d76 100644
--- a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
+++ b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
@@ -308,7 +308,7 @@ function warnUnknownProperties(type, props, eventRegistry) {
 }
 
 export function validateProperties(type, props, eventRegistry) {
-  if (isCustomElement(type, props)) {
+  if (isCustomElement(type, props) || typeof props.is === 'string') {
     return;
   }
   warnUnknownProperties(type, props, eventRegistry);
diff --git a/packages/react-dom-bindings/src/shared/isCustomElement.js b/packages/react-dom-bindings/src/shared/isCustomElement.js
index f3e03e000030c..39d5d7fe187e0 100644
--- a/packages/react-dom-bindings/src/shared/isCustomElement.js
+++ b/packages/react-dom-bindings/src/shared/isCustomElement.js
@@ -9,7 +9,7 @@
 
 function isCustomElement(tagName: string, props: Object): boolean {
   if (tagName.indexOf('-') === -1) {
-    return typeof props.is === 'string';
+    return false;
   }
   switch (tagName) {
     // These are reserved SVG and MathML elements.
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
index 75c5dd361353a..bba88467587d1 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
@@ -669,8 +669,14 @@ describe('ReactDOMServerIntegration', () => {
       }
     });
 
-    itRenders('htmlFor attribute on custom elements', async render => {
+    itRenders('htmlFor property on is elements', async render => {
       const e = await render(<div is="custom-element" htmlFor="test" />);
+      expect(e.getAttribute('htmlFor')).toBe(null);
+      expect(e.getAttribute('for')).toBe('test');
+    });
+
+    itRenders('htmlFor attribute on custom elements', async render => {
+      const e = await render(<custom-element htmlFor="test" />);
       expect(e.getAttribute('htmlFor')).toBe('test');
       expect(e.getAttribute('for')).toBe(null);
     });