Groovy inject works locally but not on Jenkins?

Question

I have the following script in a Jenkins () pipeline:

def imageTags = []
def semVerMatch = 'v0.0.0' =~ /(\d+(?:\.\d+)*)(-.*)?$/
if (semVerMatch.find()) {
    List<String> versionNumber = semVerMatch[0][1].split("\\.") // e.g. 1.2.3
    String versionSuffix = semVerMatch[0][2] ?: '' // e.g. -rc
    imageTags += versionNumber[1,-1].inject([versionNumber[0]]) { acc, val ->
        acc += "${acc[-1]}.${val}"
    }.collect { "${it}${versionSuffix}" }
}

Running locally with the same Groovy version that seems to be used on that Jenkins (Groovy Shell (2.4.13, JVM: 11.0.12), oh my 😨), I get:

===> [0, 0.0, 0.0.0]

for imageTags, as expected.

On Jenkins, however, I get an error:

java.lang.NullPointerException: Cannot invoke method getAt() on null object
(Stacktrace useless, as per usual)

By println debugging (no idea how else to proceed), I have determined that acc == null in the second "iteration" of inject:

Why? And how do I fix it?

Answer 1

On a whim I tried adding a final expression to the closure that definitely returns the new value of acc:

imageTags += versionNumber[1,-1].inject([versionNumber[0]]) { acc, val ->
    acc += "${acc[-1]}.${val}"
    acc
}.collect { "${it}${versionSuffix}" }

For some reason, that works: