diff --git a/test/IRGen/c_layout.sil b/test/IRGen/c_layout.sil
index 24e6b7d4ee9ff..4e136207fc845 100644
--- a/test/IRGen/c_layout.sil
+++ b/test/IRGen/c_layout.sil
@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -I %S/Inputs/abi %s -emit-ir | FileCheck %s --check-prefix=CHECK-%target-cpu
+// RUN: %target-swift-frontend -Xcc -fsigned-char -I %S/Inputs/abi %s -emit-ir | FileCheck %s --check-prefix=CHECK-%target-cpu
 
 sil_stage canonical
 import c_layout