diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index a9a1800af53..975f7faf968 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -17821,7 +17821,16 @@ aarch64_process_target_attr (tree args)
       num_attrs++;
       if (!aarch64_process_one_target_attr (token))
 	{
-	  error ("pragma or attribute %<target(\"%s\")%> is not valid", token);
+	  /* Check if token is possibly an arch extension without
+	     leading '+'.  */
+	  char *str = (char *) xmalloc (strlen (token) + 2);
+	  str[0] = '+';
+	  strcpy(str + 1, token);
+	  if (aarch64_handle_attr_isa_flags (str))
+	    error("arch extension %<%s%> should be prepended with %<+%>", token);
+	  else
+	    error ("pragma or attribute %<target(\"%s\")%> is not valid", token);
+	  free (str);
 	  return false;
 	}