Unable to use constants as data input for throwing events

When trying to use constants (e.g. a string "foo") as data input for throwing events (e.g. a throwing signal event), jBPM will complain with following error:

{{[KBase: defaultKieBase]: Process Compilation error F0C6036E cannot be resolved to a variable
Syntax error on token "B4B", delete this token
Syntax error on token "B", delete this token
Syntax error on tokens, delete these tokens
F0C6036E cannot be resolved to a variable
Syntax error on token "B4B", delete this token
Syntax error on token "B", delete this token
Syntax error on tokens, delete these tokens}}

It happens both in old and new BPMN Designer.