diff --git a/nptl_db/thread_dbP.h b/nptl_db/thread_dbP.h
index 8845722109..b52c254300 100644
--- a/nptl_db/thread_dbP.h
+++ b/nptl_db/thread_dbP.h
@@ -61,7 +61,9 @@ enum
 
 /* Comment out the following for less verbose output.  */
 #ifndef NDEBUG
-# define LOG(c) if (__td_debug) write (2, c "\n", strlen (c "\n"))
+# define LOG(c) \
+  if (__td_debug) \
+    assert (write (2, c "\n", strlen (c "\n")) == strlen (c "\n"))
 extern int __td_debug attribute_hidden;
 #else
 # define LOG(c)