Shell path is incorrect or not defined

At present I am having a very specific problem. When trying to Deploy an Agent in Oracle Enterprise Manager Cloud Control 13c (OEM 13c), I get the following error;

Shell path is incorrect or not defined.:C:/cygwin/bin/sh.exe(SH_PATH),-c(SH_ARGS) on host blah_blah

My environment is all Windows 2012 R2. OEM 13c, 12c R2 DB.

I have installed Cygwin SSHD on the target server, and I can "putty" to it using ssh, with the credentials I have established which is a domain user. If I use bad credentials (I forgot the user name was case sensitive) I get a different error of too many failed attempts.