substring statement is not working on my column formatting

On a sharepoint list on a modern site there is a single text field with the following data :

test~substring

I have added the below JSON to format this column to show the text 'test' but the substring does not seem to be recognised and I only see the text from within the substring :=substring(@currentField, 0, indexof(@currentField,'~')) 

Can anybody please help with this behaviour 

{
"$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",

"elmType": "span",
"txtContent": "=substring(@currentfield, 0, indexof(@currentfield,'~'))",
}