diff --git a/libs/binder/IPCThreadState.cpp b/libs/binder/IPCThreadState.cpp
index 22f6f54..66117bf 100644
--- a/libs/binder/IPCThreadState.cpp
+++ b/libs/binder/IPCThreadState.cpp
@@ -113,7 +113,7 @@
 
 static const char* getReturnString(uint32_t cmd)
 {
-    size_t idx = cmd & 0xff;
+    size_t idx = cmd & _IOC_NRMASK;
     if (idx < sizeof(kReturnStrings) / sizeof(kReturnStrings[0]))
         return kReturnStrings[idx];
     else