diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 8a1b9015150..3e8d5b86af7 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9338,7 +9338,7 @@ namespace ts {
                     }
                     if (discriminant) {
                         // check excess properties against discriminant type only, not the entire union
-                        return hasExcessProperties(source, discriminant, undefined, reportErrors);
+                        return hasExcessProperties(source, discriminant, /*discriminant*/ undefined, reportErrors);
                     }
                     for (const prop of getPropertiesOfObjectType(source)) {
                         if (!isKnownProperty(target, prop.escapedName, isComparingJsxAttributes)) {