diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index df95da23cf..ee17d6b7ac 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -593,7 +593,7 @@ pub const VTable = struct {
         context: []const u8,
         context_alignment: std.mem.Alignment,
         start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-    ) ?*AnyFuture,
+    ) error{OutOfMemory}!*AnyFuture,
     /// Returning `null` indicates resource allocation failed.
     ///
     /// Thread-safe.
@@ -606,7 +606,7 @@ pub const VTable = struct {
         context: []const u8,
         context_alignment: std.mem.Alignment,
         start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-    ) ?*AnyFuture,
+    ) error{OutOfMemory}!*AnyFuture,
     /// Executes `start` asynchronously in a manner such that it cleans itself
     /// up. This mode does not support results, await, or cancel.
     ///
@@ -1220,7 +1220,7 @@ pub fn asyncConcurrent(
         }
     };
     var future: Future(Result) = undefined;
-    future.any_future = io.vtable.asyncConcurrent(
+    future.any_future = try io.vtable.asyncConcurrent(
         io.userdata,
         @sizeOf(Result),
         .of(Result),
@@ -1231,14 +1231,14 @@ pub fn asyncConcurrent(
     return future;
 }
 
-/// Calls `function` with `args`, such that the return value of the function is
-/// not guaranteed to be available until `await` is called, while simultaneously
-/// passing control flow back to the caller.
+/// Simultaneously calls `function` with `args` while passing control flow back
+/// to the caller. The return value of the function is not guaranteed to be
+/// available until `await` is called.
 ///
 /// This has the strongest guarantees of all async family functions, placing
 /// the most restrictions on what kind of `Io` implementations are supported.
-/// By calling `asyncConcurrent` instead, one allows, for example,
-/// stackful single-threaded non-blocking I/O.
+/// By calling `asyncConcurrent` instead, one allows, for example, stackful
+/// single-threaded non-blocking I/O.
 ///
 /// See also:
 /// * `asyncConcurrent`
@@ -1258,9 +1258,9 @@ pub fn asyncParallel(
         }
     };
     var future: Future(Result) = undefined;
-    future.any_future = io.vtable.asyncConcurrent(
+    future.any_future = try io.vtable.asyncParallel(
         io.userdata,
-        @ptrCast((&future.result)[0..1]),
+        @sizeOf(Result),
         .of(Result),
         @ptrCast((&args)[0..1]),
         .of(Args),
diff --git a/lib/std/Io/EventLoop.zig b/lib/std/Io/EventLoop.zig
index 3f070362e5..8b8de73b23 100644
--- a/lib/std/Io/EventLoop.zig
+++ b/lib/std/Io/EventLoop.zig
@@ -879,7 +879,7 @@ fn async(
     context_alignment: Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
 ) ?*std.Io.AnyFuture {
-    return asyncConcurrent(userdata, result.len, result_alignment, context, context_alignment, start) orelse {
+    return asyncConcurrent(userdata, result.len, result_alignment, context, context_alignment, start) catch {
         start(context.ptr, result.ptr);
         return null;
     };
@@ -892,14 +892,14 @@ fn asyncConcurrent(
     context: []const u8,
     context_alignment: Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) ?*std.Io.AnyFuture {
+) error{OutOfMemory}!*std.Io.AnyFuture {
     assert(result_alignment.compare(.lte, Fiber.max_result_align)); // TODO
     assert(context_alignment.compare(.lte, Fiber.max_context_align)); // TODO
     assert(result_len <= Fiber.max_result_size); // TODO
     assert(context.len <= Fiber.max_context_size); // TODO
 
     const event_loop: *EventLoop = @alignCast(@ptrCast(userdata));
-    const fiber = Fiber.allocate(event_loop) catch return null;
+    const fiber = try Fiber.allocate(event_loop);
     std.log.debug("allocated {*}", .{fiber});
 
     const closure: *AsyncClosure = .fromFiber(fiber);
@@ -945,7 +945,7 @@ fn asyncParallel(
     context: []const u8,
     context_alignment: Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) ?*std.Io.AnyFuture {
+) error{OutOfMemory}!*std.Io.AnyFuture {
     _ = userdata;
     _ = result_len;
     _ = result_alignment;
diff --git a/lib/std/Io/ThreadPool.zig b/lib/std/Io/ThreadPool.zig
index c5fc45ad91..2f10c6a8fa 100644
--- a/lib/std/Io/ThreadPool.zig
+++ b/lib/std/Io/ThreadPool.zig
@@ -220,7 +220,7 @@ fn async(
     }
     const pool: *Pool = @alignCast(@ptrCast(userdata));
     const cpu_count = pool.cpu_count catch {
-        return asyncParallel(userdata, result.len, result_alignment, context, context_alignment, start) orelse {
+        return asyncParallel(userdata, result.len, result_alignment, context, context_alignment, start) catch {
             start(context.ptr, result.ptr);
             return null;
         };
@@ -291,8 +291,8 @@ fn asyncParallel(
     context: []const u8,
     context_alignment: std.mem.Alignment,
     start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) ?*Io.AnyFuture {
-    if (builtin.single_threaded) return null;
+) error{OutOfMemory}!*Io.AnyFuture {
+    if (builtin.single_threaded) unreachable;
 
     const pool: *Pool = @alignCast(@ptrCast(userdata));
     const cpu_count = pool.cpu_count catch 1;
@@ -300,7 +300,7 @@ fn asyncParallel(
     const context_offset = context_alignment.forward(@sizeOf(AsyncClosure));
     const result_offset = result_alignment.forward(context_offset + context.len);
     const n = result_offset + result_len;
-    const closure: *AsyncClosure = @alignCast(@ptrCast(gpa.alignedAlloc(u8, .of(AsyncClosure), n) catch return null));
+    const closure: *AsyncClosure = @alignCast(@ptrCast(try gpa.alignedAlloc(u8, .of(AsyncClosure), n)));
 
     closure.* = .{
         .func = start,
@@ -324,7 +324,7 @@ fn asyncParallel(
     pool.threads.ensureTotalCapacity(gpa, thread_capacity) catch {
         pool.mutex.unlock();
         closure.free(gpa, result_len);
-        return null;
+        return error.OutOfMemory;
     };
 
     pool.run_queue.prepend(&closure.runnable.node);
@@ -334,7 +334,7 @@ fn asyncParallel(
             assert(pool.run_queue.popFirst() == &closure.runnable.node);
             pool.mutex.unlock();
             closure.free(gpa, result_len);
-            return null;
+            return error.OutOfMemory;
         };
         pool.threads.appendAssumeCapacity(thread);
     }