diff --git a/pb_decode.c b/pb_decode.c
index d4ad5a3..84b0f3e 100644
--- a/pb_decode.c
+++ b/pb_decode.c
@@ -139,7 +139,7 @@
     return true;    
 }
 
-pb_istream_t pb_istream_from_buffer(const pb_byte_t *buf, size_t bufsize)
+pb_istream_t pb_istream_from_buffer(const pb_byte_t *buf, size_t msglen)
 {
     pb_istream_t stream;
     /* Cast away the const from buf without a compiler error.  We are
@@ -156,7 +156,7 @@
 #endif
     state.c_state = buf;
     stream.state = state.state;
-    stream.bytes_left = bufsize;
+    stream.bytes_left = msglen;
 #ifndef PB_NO_ERRMSG
     stream.errmsg = NULL;
 #endif
diff --git a/pb_decode.h b/pb_decode.h
index b64d95a..9b70c8b 100644
--- a/pb_decode.h
+++ b/pb_decode.h
@@ -122,10 +122,13 @@
 
 /* Create an input stream for reading from a memory buffer.
  *
+ * msglen should be the actual length of the message, not the full size of
+ * allocated buffer.
+ *
  * Alternatively, you can use a custom stream that reads directly from e.g.
  * a file or a network socket.
  */
-pb_istream_t pb_istream_from_buffer(const pb_byte_t *buf, size_t bufsize);
+pb_istream_t pb_istream_from_buffer(const pb_byte_t *buf, size_t msglen);
 
 /* Function to read from a pb_istream_t. You can use this if you need to
  * read some custom header data, or to read data in field callbacks.