Your question got me thinking that maybe there is a dependency issue because I had some other libraries that use POI included. I removed those and ran the function again and I got the following error: Execution error (NullPointerException) at http://tech.v3.dataset.io.spreadsheet/sheet->dataset (spreadsheet.clj:59). null

The only library I have included in this test project is tech.ml.dataset "6.087"

The workbook has around 8 worksheets (tabs)

It reminds me the https://clojurians.slack.com/archives/C053AK3F9/p1654262783523279 Anyway, you should be able to see the whole stracktrace to locate the exact source of the NPE.

Execution error (NullPointerException) at tech.v3.dataset.io.spreadsheet/sheet->dataset (spreadsheet.clj:59).
null
Syntax error macroexpanding at (core.clj:1:149).
	at clojure.lang.Compiler$InvokeExpr.eval(Compiler.java:3707)
java.lang.NullPointerException
	at clojure.lang.RT.longCast(RT.java:1284)
	at tech.v3.dataset.io.spreadsheet$sheet__GT_dataset.invokeStatic(spreadsheet.clj:59)
	at tech.v3.dataset.io.spreadsheet$sheet__GT_dataset.invoke(spreadsheet.clj:14)
	at tech.v3.libs.poi$workbook__GT_datasets$fn__16636.invoke(poi.clj:153)
	at clojure.core$mapv$fn__8445.invoke(core.clj:6912)
	at clojure.core.protocols$iter_reduce.invokeStatic(protocols.clj:49)
	at clojure.core.protocols$fn__8140.invokeStatic(protocols.clj:75)
	at clojure.core.protocols$fn__8140.invoke(protocols.clj:75)
	at clojure.core.protocols$fn__8088$G__8083__8101.invoke(protocols.clj:13)
	at clojure.core$reduce.invokeStatic(core.clj:6828)
	at clojure.core$mapv.invokeStatic(core.clj:6903)
	at clojure.core$mapv.invoke(core.clj:6903)
	at tech.v3.libs.poi$workbook__GT_datasets.invokeStatic(poi.clj:153)
	at tech.v3.libs.poi$workbook__GT_datasets.invoke(poi.clj:138)
	at clojure.lang.AFn.applyToHelper(AFn.java:156)
	at clojure.lang.AFn.applyTo(AFn.java:144)
	at clojure.lang.Compiler$InvokeExpr.eval(Compiler.java:3702)
	at clojure.lang.Compiler$DefExpr.eval(Compiler.java:457)
	at clojure.lang.Compiler.eval(Compiler.java:7182)
	at clojure.lang.Compiler.eval(Compiler.java:7132)
	at clojure.core$eval.invokeStatic(core.clj:3214)
	at clojure.core$eval.invoke(core.clj:3210)
	at clojure.main$repl$read_eval_print__9086$fn__9089.invoke(main.clj:437)
	at clojure.main$repl$read_eval_print__9086.invoke(main.clj:437)
	at clojure.main$repl$fn__9095.invoke(main.clj:458)
	at clojure.main$repl.invokeStatic(main.clj:458)
	at clojure.main$repl.doInvoke(main.clj:368)
	at clojure.lang.RestFn.invoke(RestFn.java:1523)
	at nrepl.middleware.interruptible_eval$evaluate.invokeStatic(interruptible_eval.clj:79)
	at nrepl.middleware.interruptible_eval$evaluate.invoke(interruptible_eval.clj:55)
	at nrepl.middleware.interruptible_eval$interruptible_eval$fn__17428$fn__17432.invoke(interruptible_eval.clj:142)
	at clojure.lang.AFn.run(AFn.java:22)
	at nrepl.middleware.session$session_exec$main_loop__17529$fn__17533.invoke(session.clj:171)
	at nrepl.middleware.session$session_exec$main_loop__17529.invoke(session.clj:170)
	at clojure.lang.AFn.run(AFn.java:22)
	at java.base/java.lang.Thread.run(Thread.java:829)

So, the first error was on a windows machine. I did not receive this error using my linux machine. However, I did run into a Java Heap space error (probably because of the size of the worksheets). Any way around this?

If it's legitimate then increase max heap aize via -Xmx

Can you file an issue with a very small spreadsheet attached that exhibits the load failure? The OOM issue is next - the dataset representation is far smaller than the spreadsheet so one thought I have is process each sheet one at a time making sure to release any references to the workbook after you creat the sequence of datasets

Sure, can provide two versions of the spreadsheet. One version will have an excel addin that will probably give you lot of login prompts; this is because the data is being pulled from an API using this addin. The other won't have the addin activated. I just want to make sure I provide you a realistic example of my environment.

That addin - does it work with poi?