diff --git a/CHANGELOG.md b/CHANGELOG.md
index abffcbc..6e2d3e6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,11 +22,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Breaking:** after `open FSharp.Control.R3`, the bare type name `Observable` refers to `FSharp.Control.R3.ObservableFactories.Observable`; write `open type R3.Observable` and extend `R3.Observable` by its full name
 - **Breaking:** `rxquery` `sumBy` adds values of the projected type: it requires `(+) : 'Value * 'Value -> 'Value`
 - `mapAsync` and `iterAsync` validate the concurrency limit of the options when they are called
+- `chunkBySize` and `chunkBy` validate every chunk length when they are called and report the parameter and the rejected value
 
 ### Fixed
 
 - `rxquery` emitted its elements on the thread pool, out of order and after the source had moved on; `yield` and `zero` are now synchronous
 - `rxquery` `sumBy` passed `null` to the `(+)` of reference types
+- `chunkBy` accepted a non-positive window length of `ChunkTimeSpanCount` and `ChunkMillisecondsCount`, which failed every element
 
 ## [0.3.1] - 2026-01-28
 
diff --git a/src/FSharp.Control.R3/Observable.fs b/src/FSharp.Control.R3/Observable.fs
index 4d49ef3..c332b57 100644
--- a/src/FSharp.Control.R3/Observable.fs
+++ b/src/FSharp.Control.R3/Observable.fs
@@ -22,22 +22,43 @@ let inline catch ([<InlineIfLambda>] f : 'Exn -> Observable<'T>) o = ObservableE
 /// upn the successful termination of the first
 let inline concat first second = ObservableExtensions.Concat (first, second)
 
-///<summary>Divides the input observable sequence into chunks of size at most <c>chunkSize</c>.</summary>
-///<param name="chunkSize">The maximum size of each chunk.</param>
-///<param name="source">The input observable sequence.</param>
-///<returns>The observable sequence divided into chunks.</returns>
-///<exception cref="T:System.ArgumentNullException">Thrown when the input sequence is null.</exception>
-///<exception cref="T:System.ArgumentException">Thrown when <c>chunkSize</c> is not positive.</exception>
-let inline chunkBySize (chunkSize : int) (source) = ObservableExtensions.Chunk (source, chunkSize)
-
-let inline chunkBy (configuration : ChunkConfiguration<'T>) (source) =
+// R3 validates the length of Chunk(count) with the message as the parameter name and without the value, and does not validate
+// the time-and-count overloads at all: there 0 makes every element fail with an IndexOutOfRangeException through OnErrorResume
+// and a negative length fails only at subscription. The library validates every length itself, the same way.
+let private ensurePositiveLength (paramName : string) (length : int) =
+    if length <= 0 then
+        raise (ArgumentOutOfRangeException (paramName, length, "The chunk length must be positive."))
+
+/// <summary>Divides the source into chunks of at most <paramref name="chunkSize"/> elements.</summary>
+/// <param name="chunkSize">The maximum size of each chunk.</param>
+/// <param name="source">The input observable sequence.</param>
+/// <returns>
+/// The observable sequence divided into chunks. When the source completes, successfully or not,
+/// the remaining elements are emitted as a last, shorter chunk.
+/// </returns>
+/// <exception cref="T:System.ArgumentOutOfRangeException">Thrown when <paramref name="chunkSize"/> is not positive.</exception>
+let chunkBySize (chunkSize : int) (source : Observable<'T>) =
+    ensurePositiveLength (nameof chunkSize) chunkSize
+    ObservableExtensions.Chunk (source, chunkSize)
+
+/// <summary>
+/// Divides the source into chunks as defined by <paramref name="configuration"/>.
+/// When the source completes, the buffered elements are emitted as a last chunk.
+/// </summary>
+/// <exception cref="T:System.ArgumentOutOfRangeException">Thrown when the window length of the configuration is not positive.</exception>
+let chunkBy (configuration : ChunkConfiguration<'T>) (source : Observable<'T>) =
     match configuration with
-    | ChunkCount count -> ObservableExtensions.Chunk (source, count)
+    | ChunkCount count ->
+        ensurePositiveLength (nameof configuration) count
+        ObservableExtensions.Chunk (source, count)
     | ChunkTimeSpan (timeSpan, timeProvider) -> ObservableExtensions.Chunk (source, timeSpan, timeProvider)
-    | ChunkTimeSpanCount (timeSpan, count, timeProvider) -> ObservableExtensions.Chunk (source, timeSpan, count, timeProvider)
+    | ChunkTimeSpanCount (timeSpan, count, timeProvider) ->
+        ensurePositiveLength (nameof configuration) count
+        ObservableExtensions.Chunk (source, timeSpan, count, timeProvider)
     | ChunkMilliseconds (milliseconds, timeProvider) ->
         ObservableExtensions.Chunk (source, TimeSpan.FromMilliseconds (float milliseconds), timeProvider)
     | ChunkMillisecondsCount (milliseconds, count, timeProvider) ->
+        ensurePositiveLength (nameof configuration) count
         ObservableExtensions.Chunk (source, TimeSpan.FromMilliseconds (float milliseconds), count, timeProvider)
     | ChunkAsyncWindow (asyncWindow, configureAwait) -> ObservableExtensions.Chunk (source, asyncWindow, configureAwait)
     | ChunkWindowBoundaries windowBoundaries -> ObservableExtensions.Chunk (source, windowBoundaries = windowBoundaries)