diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d51925b0459d8..996d2738a9882 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -32557,10 +32557,10 @@ namespace ts {
             //
             // The result is always of the Boolean primitive type.
             const rightTypeConstraint = getConstraintOfType(rightType);
-            if (!allTypesAssignableToKind(rightType, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive) ||
+            if (!allTypesAssignableToKind(rightType, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive | TypeFlags.Unknown) ||
                 rightTypeConstraint && (
-                    isTypeAssignableToKind(rightType, TypeFlags.UnionOrIntersection) && !allTypesAssignableToKind(rightTypeConstraint, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive) ||
-                    !maybeTypeOfKind(rightTypeConstraint, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive | TypeFlags.Object)
+                    isTypeAssignableToKind(rightType, TypeFlags.UnionOrIntersection) && !allTypesAssignableToKind(rightTypeConstraint, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive | TypeFlags.Unknown) ||
+                    !maybeTypeOfKind(rightTypeConstraint, TypeFlags.NonPrimitive | TypeFlags.InstantiableNonPrimitive | TypeFlags.Unknown | TypeFlags.Object)
                 )
             ) {
                 error(right, Diagnostics.The_right_hand_side_of_an_in_expression_must_not_be_a_primitive);