MovieClip custom property lost in onRelease func?

I’m trying to assign a custom property to an attached MC. In this case it’s an ID number, but it’s always undefined inside the onRelease function… What am I missing?

 ActionScript Code:
 [FONT=Courier New][LEFT][COLOR=#000000]**function**[/COLOR] blablaMethod[COLOR=#000000]([/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000]{[/COLOR][INDENT][COLOR=#0000ff]_root[/COLOR].[COLOR=#0000ff]attachMovie[/COLOR][COLOR=#000000]([/COLOR][COLOR=#ff0000]"bla"[/COLOR], [COLOR=#ff0000]"bla"[/COLOR]+someIncrementingId, [COLOR=#0000ff]_root[/COLOR].[COLOR=#0000ff]getNextHighestDepth[/COLOR][COLOR=#000000]([/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000])[/COLOR];

[/INDENT][INDENT][COLOR=#000000]var[/COLOR] mcReference = [COLOR=#0000ff]_root[/COLOR][COLOR=#000000][[/COLOR][COLOR=#ff0000]“bla”[/COLOR]+someIncrementingId[COLOR=#000000]][/COLOR];
[/INDENT][INDENT][COLOR=#808080]// I’ve also tried the following:[/COLOR]
[/INDENT][INDENT][COLOR=#000000]var[/COLOR] mcReference = [COLOR=#0000ff]_root[/COLOR].[COLOR=#0000ff]attachMovie[/COLOR][COLOR=#000000]([/COLOR][COLOR=#ff0000]“bla”[/COLOR], [COLOR=#ff0000]“bla”[/COLOR]+[COLOR=#0000ff]number[/COLOR], [COLOR=#0000ff]_root[/COLOR].[COLOR=#0000ff]getNextHighestDepth[/COLOR]COLOR=#000000[/COLOR][COLOR=#000000])[/COLOR];
mcReference.[COLOR=#000080]idNum[/COLOR] = someNumber;
[COLOR=#808080]// some code for positioning the clip[/COLOR]
[COLOR=#808080]// some more code, then:[/COLOR]
[/INDENT][INDENT]mcReference.[COLOR=#0000ff]onRelease[/COLOR] = [COLOR=#000000]function[/COLOR]COLOR=#000000[/COLOR][COLOR=#000000]{[/COLOR][INDENT][COLOR=#808080]// this traces undefined:[/COLOR]
[COLOR=#0000ff]trace[/COLOR]COLOR=#000000[/COLOR];
[COLOR=#808080]// this as well:[/COLOR]
[COLOR=#0000ff]trace[/COLOR]COLOR=#000000[/COLOR];
[COLOR=#808080]// and this:[/COLOR]
[COLOR=#0000ff]trace[/COLOR]COLOR=#000000[/COLOR];
[/INDENT][/INDENT][INDENT][COLOR=#000000]}[/COLOR]
[/INDENT][COLOR=#000000]}

[/COLOR] [/LEFT]
[/FONT]