Index: generic/tclIO.c
==================================================================
--- generic/tclIO.c
+++ generic/tclIO.c
@@ -4271,10 +4271,11 @@
 	if (written < 0) {
 	    return TCL_INDEX_NONE;
 	}
 	totalWritten += written;
 	srcLen -= chunkSize;
+        src += written;
     } while (srcLen);
     return totalWritten;
 }
 
 static void