Invalid memory address Runtime error when trying to start Mattermost following Debian installation guide

I've followed the Debian installation guide to install Mattermost on a single VPS (1GB RAM, 30GB SSD), but I can't get Mattermost to start.

http://docs.mattermost.com/install/prod-debian.html

I'm using Mattermost v3.1.0. When I get to the step to test the Mattermost server, it fails to run as follows:

root@mattermost:/opt/mattermost/bin# ./platform 
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: interface conversion: interface is runtime.errorString, not string
[signal 0xb code=0x1 addr=0x0 pc=0x776cb2]

goroutine 1 [running]:
panic(0xcd1aa0, 0xc820015a00)
	/usr/local/go/src/runtime/panic.go:481 +0x3e6
main.doLoadConfig.func1(0xc820149d48)
	/var/lib/jenkins/jobs/mattermost-platform-release/workspace/src/github.com/mattermost/platform/mattermost.go:73 +0x6f
panic(0xccbec0, 0xc82000a0e0)
	/usr/local/go/src/runtime/panic.go:443 +0x4e9
github.com/mattermost/platform/utils.LoadConfig(0xc82012dad0, 0x22)
	/var/lib/jenkins/jobs/mattermost-platform-release/workspace/src/github.com/mattermost/platform/utils/config.go:172 +0xd12
main.doLoadConfig(0xdce990, 0xb, 0x0, 0x0)
	/var/lib/jenkins/jobs/mattermost-platform-release/workspace/src/github.com/mattermost/platform/mattermost.go:76 +0x6a
main.main()
	/var/lib/jenkins/jobs/mattermost-platform-release/workspace/src/github.com/mattermost/platform/mattermost.go:84 +0x43

I've tried searching the web on the error message, but all the results I've found have slightly different errors -- I've checked the formatting of my JSON file, testing the PostgreSQL credentials... not sure what the problem could be. Any help would be appreciated.