Access Denied while trying to create mini-dump with WinDbg

I am trying to create a mini-dump using WinDbg. After attaching to the process (non-invasive), I execute the following command:

.dump /mA c:\Temp\app.dmp

This results in the following error:

Creating c:\Temp\app.dmp - mini user dump
WriteFullMemory.Memory.Read(0xe730000, 0x10000) failed 0x8007012b, skip.
WriteFullMemory.QueryVirtual(0x0) for info failed, 0x80070005
Dump creation failed, Win32 error 0n5
    "Access is denied."

WinDdg is being run as an Administrator. We have the lastest version of WinDbg, we confirmed we're running the 64 bit version and the process being attached is 64 bit. Does anyone know how this can be resolved?

For context, we're taking the dump to isolate the root cause of a memory leak in one of our IIS-hosted WCF Web Services.

Side note: this may be due to paged memory but not sure how to get around that.