diff --git a/NoSleep.Test/ChildProcessHoldTests.cs b/NoSleep.Test/ChildProcessHoldTests.cs
index b78df33..834a125 100644
--- a/NoSleep.Test/ChildProcessHoldTests.cs
+++ b/NoSleep.Test/ChildProcessHoldTests.cs
@@ -106,6 +106,29 @@ public void Start_WhenTheHelperFloodsStandardError_KeepsOnlyTheLastOfItInTheMess
 		Assert.IsLessThan(4096 + 200, exception.Message.Length, "The message should stay bounded");
 	}
 
+	[TestMethod]
+	public void Start_WhenTheHelperExitsLeavingAChildOnItsStandardError_ThrowsWithoutWaitingForTheChild()
+	{
+		if (!IsUnix)
+		{
+			Assert.Inconclusive("The helper paths used by this test only exist on Unix.");
+			return;
+		}
+
+		// The background sleep inherits the helper's standard error, so the pipe stays open for 20 s after the
+		// helper itself has exited (#43).
+		const string Script = "echo the-real-reason >&2; sleep 20 & exit 1";
+
+		Stopwatch stopwatch = Stopwatch.StartNew();
+		SleepBlockException exception = Assert.ThrowsExactly<SleepBlockException>(
+			() => ChildProcessHold.Start(ImmediateExitHelper, ["-c", Script]));
+		stopwatch.Stop();
+
+		StringAssert.Contains(exception.Message, "exited immediately with code 1", StringComparison.Ordinal);
+		StringAssert.Contains(exception.Message, "the-real-reason", StringComparison.Ordinal);
+		Assert.IsLessThan(TimeSpan.FromSeconds(5), stopwatch.Elapsed, "Start should not wait for the helper's background child");
+	}
+
 	[TestMethod]
 	public void Start_WithAMissingHelper_Throws()
 	{
diff --git a/NoSleep/Platforms/ChildProcessHold.cs b/NoSleep/Platforms/ChildProcessHold.cs
index 21afaab..b6b700d 100644
--- a/NoSleep/Platforms/ChildProcessHold.cs
+++ b/NoSleep/Platforms/ChildProcessHold.cs
@@ -9,6 +9,7 @@ namespace ktsu.NoSleep.Platforms;
 using System.Globalization;
 using System.IO;
 using System.Threading;
+using System.Threading.Tasks;
 
 /// <summary>
 /// A sleep inhibitor that exists for as long as a helper process stays alive.
@@ -27,6 +28,16 @@ public sealed class ChildProcessHold : IDisposable
 	private static readonly TimeSpan StartupGrace = TimeSpan.FromMilliseconds(250);
 	private static readonly TimeSpan GracefulExitTimeout = TimeSpan.FromSeconds(2);
 
+	/// <summary>
+	/// How long to wait, once a helper has exited, for the rest of its standard error to arrive.
+	/// </summary>
+	/// <remarks>
+	/// The pipe reaches end of input only when every process holding its write end has closed it. A helper
+	/// that started a background child before exiting leaves that child holding the pipe, so waiting for end
+	/// of input without a deadline would wait for as long as the child runs.
+	/// </remarks>
+	private static readonly TimeSpan ExitDrainTimeout = TimeSpan.FromSeconds(1);
+
 	private readonly Process process;
 	private bool disposed;
 
@@ -96,9 +107,10 @@ public static ChildProcessHold Start(string fileName, IEnumerable<string> argume
 		if (process.WaitForExit(GetMilliseconds(StartupGrace)))
 		{
 			// The timed wait returns as soon as the process ends, possibly before its last lines of standard
-			// error have been read. The untimed wait also waits for both streams to reach end of input, which
-			// they do promptly now that the helper is gone.
-			process.WaitForExit();
+			// error have been read. Wait a bounded time for the stream to reach end of input, rather than with
+			// the untimed WaitForExit: a background child the helper left behind can hold the pipe open for as
+			// long as it runs, and this wait happens inside the blocker's and the controller's locks.
+			_ = startupErrors.WaitForEndOfInput(ExitDrainTimeout);
 			int exitCode = process.ExitCode;
 			string errorOutput = startupErrors.Stop();
 			process.Dispose();
@@ -180,6 +192,7 @@ private sealed class StartupErrorOutput
 
 		private readonly Lock gate = new();
 		private readonly LinkedList<string> lines = new();
+		private readonly TaskCompletionSource endOfInput = new(TaskCreationOptions.RunContinuationsAsynchronously);
 		private int length;
 		private bool stopped;
 
@@ -189,6 +202,12 @@ private sealed class StartupErrorOutput
 		/// <param name="line">The line, or <see langword="null"/> at end of input.</param>
 		public void Add(string? line)
 		{
+			if (line is null)
+			{
+				_ = endOfInput.TrySetResult();
+				return;
+			}
+
 			if (string.IsNullOrWhiteSpace(line))
 			{
 				return;
@@ -220,6 +239,13 @@ public void Add(string? line)
 			}
 		}
 
+		/// <summary>
+		/// Waits for standard error to reach end of input, up to a deadline.
+		/// </summary>
+		/// <param name="timeout">The longest time to wait.</param>
+		/// <returns><see langword="true"/> when end of input arrived; <see langword="false"/> when the deadline passed first.</returns>
+		public bool WaitForEndOfInput(TimeSpan timeout) => endOfInput.Task.Wait(timeout);
+
 		/// <summary>
 		/// Stops recording and returns what was recorded.
 		/// </summary>