diff --git a/Help/command/add_custom_command.rst b/Help/command/add_custom_command.rst
index 9fbad4bf6..1307a58c5 100644
--- a/Help/command/add_custom_command.rst
+++ b/Help/command/add_custom_command.rst
@@ -130,7 +130,8 @@ The options are:
   Specify the primary input source file to the command.  This is
   treated just like any value given to the ``DEPENDS`` option
   but also suggests to Visual Studio generators where to hang
-  the custom command.
+  the custom command.  At most one custom command may specify a
+  given source file as its main dependency.
 
 ``OUTPUT``
   Specify the output files the command is expected to produce.