diff --git a/base/HidlSupport.cpp b/base/HidlSupport.cpp
index 8f3c057..a69faa2 100644
--- a/base/HidlSupport.cpp
+++ b/base/HidlSupport.cpp
@@ -254,6 +254,14 @@
     if (size > UINT32_MAX) {
         LOG(FATAL) << "string size can't exceed 2^32 bytes: " << size;
     }
+
+    // When the binder driver copies this data into its buffer, it must
+    // have a zero byte there because the remote process will have a pointer
+    // directly into the read-only binder buffer. If we manually copy the
+    // data now to add a zero, then we lose the efficiency of this method.
+    // Checking here (it's also checked in the parceling code later).
+    CHECK(data[size] == '\0');
+
     clear();
 
     mBuffer = data;
diff --git a/base/include/hidl/HidlSupport.h b/base/include/hidl/HidlSupport.h
index f09eb63..d1221fe 100644
--- a/base/include/hidl/HidlSupport.h
+++ b/base/include/hidl/HidlSupport.h
@@ -155,6 +155,8 @@
     // Reference an external char array. Ownership is _not_ transferred.
     // Caller is responsible for ensuring that underlying memory is valid
     // for the lifetime of this hidl_string.
+    //
+    // size == strlen(data)
     void setToExternal(const char *data, size_t size);
 
     // offsetof(hidl_string, mBuffer) exposed since mBuffer is private.