diff --git a/include/swift/AST/DiagnosticsSIL.def b/include/swift/AST/DiagnosticsSIL.def
index 5835a6d5075..88f74ba7bc6 100644
--- a/include/swift/AST/DiagnosticsSIL.def
+++ b/include/swift/AST/DiagnosticsSIL.def
@@ -221,7 +221,7 @@ ERROR(missing_never_call,none,
       "call to another never-returning function on all paths",
       (Type, unsigned))
 ERROR(guard_body_must_not_fallthrough,none,
-      "'guard' body may not fall through, consider using a 'return' or 'throw'"
+      "'guard' body must not fall through, consider using a 'return' or 'throw'"
       " to exit the scope", ())
 WARNING(unreachable_code,none, "will never be executed", ())
 NOTE(unreachable_code_branch,none,
diff --git a/test/SILOptimizer/unreachable_code.swift b/test/SILOptimizer/unreachable_code.swift
index 608f6803292..91046450548 100644
--- a/test/SILOptimizer/unreachable_code.swift
+++ b/test/SILOptimizer/unreachable_code.swift
@@ -218,7 +218,7 @@ class r20097963MyClass {
 func die() -> Never { die() }
 
 func testGuard(_ a : Int) {
-  guard case 4 = a else {  }  // expected-error {{'guard' body may not fall through, consider using a 'return' or 'throw'}}
+  guard case 4 = a else {  }  // expected-error {{'guard' body must not fall through, consider using a 'return' or 'throw'}}
 
   guard case 4 = a else { return }  // ok
   guard case 4 = a else { die() }  // ok