diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 2e7edd7..8e206f5 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -2013,8 +2013,8 @@
 
 const char* Parcel::readCString() const
 {
-    const size_t avail = mDataSize-mDataPos;
-    if (avail > 0) {
+    if (mDataPos < mDataSize) {
+        const size_t avail = mDataSize-mDataPos;
         const char* str = reinterpret_cast<const char*>(mData+mDataPos);
         // is the string's trailing NUL within the parcel's valid bounds?
         const char* eos = reinterpret_cast<const char*>(memchr(str, 0, avail));