DPath only allows predicates as integer expressions to index arrays. With an invalid test, a Schema Definition Error is thrown but it seems to be trying to parse the predicate as a path and giving the error message that the element was not found.

[error] Test edu.illinois.ncsa.daffodil.section23.dfdl_expressions.TestDFDLExpressionsDebug.test_predicate_02 failed: 

java.lang.Exception: Schema Definition Error: No element corresponding to step b found.

See tests:
test_predicate_02
test_predicate_03

In files:
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section23/dfdl_expressions/TestDFDLExpressionsDebug.scala
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section23/dfdl_expressions/expressions.tdml