Netexpress 5.1 hangs on CLOSE statement

I have a program I'm writing and testing under NE5.1. I have an ISAM file that is to be shared between a number of workstations. This will be achieved simply by mapping a drive letter to something like:

NET USE E: \\BILLSPC\C$\SHAREDORDERS\ /u=blah blah, etc

Each workstation will open a file: E:ORDERS.DAT.

This is working well and all functionality tested ok. I need to check what is going to happen if/when the network is down so what I do is:

1. Map the drive

2. Start the system in Netexpress and create some data

3. issue a NET USE E: /D command to remove the drive

4. Within the still-running Netexpress system, run a program that firstly tries to CLOSE this file.

At this point, the program simply hangs on the CLOSE. No error is returning, it just hangs. Declaratives are not executed. When I select to stop animating I get a message: Execution of program xyz has been interrupted. This program is not animatable, but you can view the call/perform stack. The cause of the interrupt was 114 Attempt to access item beyond bounds of memory.

I thought a CLOSE statement simply did a logical thing these days and not a physical act (like communication with the outside world).