--- a/xpcom/base/moz.build
+++ b/xpcom/base/moz.build
@@ -120,17 +120,17 @@ UNIFIED_SOURCES += [
 ]
 
 # MOZ_STACKWALKING is defined in configure.in when the build configuration meets
 # the conditions for NS_StackWalk to work correctly.
 # We exclude this file from other build configurations so that if somebody adds a
 # new usage of NS_StackWalk it will cause a link error, which is better than having
 # NS_StackWalk silently return garbage at runtime.
 if CONFIG['MOZ_STACKWALKING']:
-    UNIFIED_SOURCES += [
+    SOURCES += [
         'nsStackWalk.cpp',
     ]
 
 if CONFIG['OS_ARCH'] == 'Linux':
     SOURCES += [
         'LinuxUtils.cpp',
         'SystemMemoryReporter.cpp',
     ]