Cisco 2911 IOS Upgrade Error

[ALIAOF_]

I'm trying to tftp new IOS to 2911 router but I keep getting this error:

%Warning: File not a valid executable for this system

This is the IOS I am trying to load: c2900-universalk9-mz.SPA.152-4.M6a.bin

Anyone encountered this before?

[Leo Laohoo]

Never seen this error message before.  Did you try to compare the MD5 hash value?

[ALIAOF_]

Ok so weirdest thing.  It wouldn't work from the LAN, but I was able to load it over the WAN.  Took forever but once it was on the router I set the router to boot from the new image, router came back online with the new image successfully.

[Akash Agrawal]

I tried to download an image of router 1841 on router 2851 and getting same error. Please check if you are downloading a valid file. If that does not help, please upload show tech of the router.

R11_2851#copy tftp flash:                
Address or name of remote host [10.76.76.160]? 
Source filename [c1841-advsecurityk9-mz.124-24.T5]? c1841-advsecurityk9-mz.124-24.T5.bin
Destination filename [c1841-advsecurityk9-mz.124-24.T5.bin]? 
Accessing tftp://10.76.76.160/c1841-advsecurityk9-mz.124-24.T5.bin...
%Warning: File not a valid executable for this system
Abort Copy? [confirm]
R11_2851#

[ALIAOF_]

It definitely is a valid IOS, it is for Cisco 2911 and is Cisco suggested.  I have loaded it up on other 2911's as well.