CQL function operand resultType null

[arby50]

When including FHIRHelpers version '4.0.1' in a CQL file, compiling into ELM and calling engine.evaluate() using the resulting ELM file, the subsequent CQLEngine.loadAndValidate() call errors when attempting to add the first ExpressionDef "ToInterval" based on the null resultType of the 'period' Operand.
CQL:
define function ToInterval(period FHIR.Period):

exception in Signature.java
throw new IllegalArgumentException("operandTypes in signatures cannot be null");

I assume this is based on a setting in cql-to-elm that would include the resultType but cannot find the setting. The ELM contains a resultTypeName only:
"resultTypeName" : "{http://hl7.org/fhir}Period",