diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 9032616e2e6..9a97e3c80c4 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -2192,7 +2192,7 @@ ERROR(attr_only_on_parameters,none,
 ERROR(attr_not_on_variadic_parameters,none,
       "%0 must not be used on variadic parameters", (StringRef))
 ERROR(attr_not_on_subscript_parameters,none,
-      "%0 may not be used on subscript parameters", (StringRef))
+      "%0 must not be used on subscript parameters", (StringRef))
 
 ERROR(override_final,none,
       "%0 overrides a 'final' %1", (DescriptiveDeclKind, DescriptiveDeclKind))
diff --git a/test/Sema/immutability.swift b/test/Sema/immutability.swift
index 84f74ae3f29..1091b5db05e 100644
--- a/test/Sema/immutability.swift
+++ b/test/Sema/immutability.swift
@@ -610,8 +610,8 @@ func f(a : FooClass, b : LetStructMembers) {
 class MutableSubscripts {
   var x : Int = 0
 
-  subscript(x: inout Int) -> () { x += 1 } // expected-error {{'inout' may not be used on subscript parameters}}
-  subscript<T>(x: inout T) -> () { // expected-error {{'inout' may not be used on subscript parameters}}
+  subscript(x: inout Int) -> () { x += 1 } // expected-error {{'inout' must not be used on subscript parameters}}
+  subscript<T>(x: inout T) -> () { // expected-error {{'inout' must not be used on subscript parameters}}
     fatalError()
   }