diff --git a/common/changes/@uifabric/utilities/keco-mark-setPortalAttribute-public_2018-09-10-21-17.json b/common/changes/@uifabric/utilities/keco-mark-setPortalAttribute-public_2018-09-10-21-17.json
new file mode 100644
index 00000000000000..281da775b60ee9
--- /dev/null
+++ b/common/changes/@uifabric/utilities/keco-mark-setPortalAttribute-public_2018-09-10-21-17.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@uifabric/utilities",
+      "comment": "Mark setPortalAttribute dom utility function as public to fix undefined runtime error.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@uifabric/utilities",
+  "email": "keco@microsoft.com"
+}
\ No newline at end of file
diff --git a/packages/utilities/src/dom.ts b/packages/utilities/src/dom.ts
index e9143b3ec597e1..0de632697c2495 100644
--- a/packages/utilities/src/dom.ts
+++ b/packages/utilities/src/dom.ts
@@ -76,10 +76,7 @@ export function getVirtualParent(child: HTMLElement): HTMLElement | undefined {
  * @public
  */
 export function getParent(child: HTMLElement, allowVirtualParents: boolean = true): HTMLElement | null {
-  return (
-    child &&
-    ((allowVirtualParents && getVirtualParent(child)) || (child.parentNode && (child.parentNode as HTMLElement)))
-  );
+  return child && ((allowVirtualParents && getVirtualParent(child)) || (child.parentNode && (child.parentNode as HTMLElement)));
 }
 
 /**
@@ -112,11 +109,7 @@ export function getChildren(parent: HTMLElement, allowVirtualChildren: boolean =
  *
  * @public
  */
-export function elementContains(
-  parent: HTMLElement | null,
-  child: HTMLElement | null,
-  allowVirtualParents: boolean = true
-): boolean {
+export function elementContains(parent: HTMLElement | null, child: HTMLElement | null, allowVirtualParents: boolean = true): boolean {
   let isContained = false;
 
   if (parent && child) {
@@ -209,6 +202,7 @@ export function getRect(element: HTMLElement | Window | null): IRectangle | unde
 /**
  * Identify element as a portal by setting an attribute.
  * @param element Element to mark as a portal.
+ * @public
  */
 export function setPortalAttribute(element: HTMLElement): void {
   element.setAttribute(DATA_PORTAL_ATTRIBUTE, 'true');
@@ -235,10 +229,7 @@ export function portalContainsElement(target: HTMLElement, parent?: HTMLElement)
  * @param matchFunction the function that determines if the element is a match
  * @returns the matched element or null no match was found
  */
-export function findElementRecursive(
-  element: HTMLElement | null,
-  matchFunction: (element: HTMLElement) => boolean
-): HTMLElement | null {
+export function findElementRecursive(element: HTMLElement | null, matchFunction: (element: HTMLElement) => boolean): HTMLElement | null {
   if (!element || element === document.body) {
     return null;
   }