diff --git a/lib/script.py b/lib/script.py
index 150eb95d48..550b04d29d 100644
--- a/lib/script.py
+++ b/lib/script.py
@@ -189,7 +189,7 @@ def generate_products(self):
     depfile = $out.d
 
 rule MergeSwiftModule
-    command = mkdir -p `dirname $out`; ${SWIFT} -frontend -emit-module $partials ${TARGET_SWIFTCFLAGS} $flags -module-cache-path ${MODULE_CACHE_PATH} -module-link-name $module_name -o $out
+    command = mkdir -p `dirname $out`; ${SWIFT} -frontend -sil-merge-partial-modules -emit-module $partials ${TARGET_SWIFTCFLAGS} $flags -module-cache-path ${MODULE_CACHE_PATH} -module-link-name $module_name -o $out
     description = Merge $out
 """