diff --git a/src/data.c b/src/data.c
index 8b9490d9466..b269ec6d501 100644
--- a/src/data.c
+++ b/src/data.c
@@ -1724,7 +1724,8 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
 
 	if (unbinding_p && blv->fwd)
 	  /* Forbid unbinding built-in variables.  */
-	  error ("Built-in variables may not be unbound");
+	  error ("Built-in variable may not be unbound : %s",
+		 SDATA (SYMBOL_NAME (symbol)));
 
 	if (NILP (where))
 	  XSETBUFFER (where, current_buffer);
@@ -1803,7 +1804,8 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
 
 	if (unbinding_p)
 	  /* Forbid unbinding built-in variables.  */
-	  error ("Built-in variables may not be unbound");
+	  error ("Built-in variable may not be unbound : %s",
+		 SDATA (SYMBOL_NAME (symbol)));
 
 	if (BUFFER_OBJFWDP (innercontents))
 	  {