diff --git a/src/lib/sort/sort-header.ts b/src/lib/sort/sort-header.ts
index 87f497d40c23..eda8f464a566 100644
--- a/src/lib/sort/sort-header.ts
+++ b/src/lib/sort/sort-header.ts
@@ -146,6 +146,6 @@ export class MatSortHeader implements MatSortable {
   /** Whether this MatSortHeader is currently sorted in either ascending or descending order. */
   _isSorted() {
     return this._sort.active == this.id &&
-        this._sort.direction === 'asc' || this._sort.direction === 'desc';
+        (this._sort.direction === 'asc' || this._sort.direction === 'desc');
   }
 }