Can't Execute Master.dbo.xp_cmdshell Throught A Variable.

Hi all.

I've previous posted a message with error building the expression of the dtexec.exe string. But now, the problem is other, and I've decided to create other threat.

Here is what I'm doing:
I've a SP that receives every parameter so I can build my dtexec.exe statement to be executed by a variable with something like this exec @rc = master.dbo.xp_cmdshell @CMD


Till now I have no problem executing it, not from a variable, but just with the statement itself, like this:
exec master.dbo.xp_cmdshell 'C:PROGRA~2MICROS~290DTSBINNDTEXEC.EXE /SQ PACKAGE /SET "Package.Variables[PERIOD].Value";20070101'.

But now, I have this SP that builds the string into a variable and then I just want to execute it throught the variable like this: exec @rc = master.dbo.xp_cmdshell @CMD.

When I make a SELECT @CMD just before the exec @rc = master.dbo.xp_cmdshell @CMD, so it prints the builded string, copy it and pasted to exec master.dbo.xp_cmdshell 'copied string' it works fine. But the step of exec @rc = master.dbo.xp_cmdshell @CMD returns the error:
The filename, directory name, or volume label syntax is incorrect.

Why is this, if I copy the generated string and execute it by the other way it works fine, but executing the generated string throught the variable it gives me that error?