Createinstallmedia will not function under Mojave

I followed the instructions in Apple's support document to the letter. I used the exact phrase:

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

But when I try to do so, I get this:

"To use this tool, you must download the macOS installer application on a Mac with 10.12.5 or later, or El Capitan 10.11.6."

I AM installing it on a Mac with 10.12.5 or later. In fact it's 10.14 (I already updated to Mojave). So why won't this work?