diff --git a/build.py b/build.py
index 23964d7d38..47321f2e8e 100755
--- a/build.py
+++ b/build.py
@@ -65,7 +65,7 @@
 ])
 
 swift_cflags += [
-	'-I${BUILD_DIR}/Foundation/usr/lib/swift',
+	'-I${BUILD_DIR}/Foundation/${PREFIX}/lib/swift',
 ]
 
 if "XCTEST_BUILD_DIR" in Configuration.current.variables: