diff --git a/CHANGELOG.md b/CHANGELOG.md
index b1b988a..82504fd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- The `tensor` module's `concat` function now returns an `AxisNotFound` error
+  when the given find function is `False` for every `Axis`.
+
 ## v0.4.0 - 2023-09-11
 
 - Argamak now requires Gleam v0.30 or later.
diff --git a/gleam.toml b/gleam.toml
index 2c5ed71..2a709e4 100644
--- a/gleam.toml
+++ b/gleam.toml
@@ -1,5 +1,5 @@
 name = "argamak"
-version = "0.4.0"
+version = "0.5.0-dev"
 description = "A tensor library for the Gleam programming language"
 licences = ["Apache-2.0"]
 gleam = "~> 0.30"
diff --git a/src/argamak/tensor.gleam b/src/argamak/tensor.gleam
index 71a1b1c..41e0a09 100644
--- a/src/argamak/tensor.gleam
+++ b/src/argamak/tensor.gleam
@@ -24,6 +24,7 @@ pub type Native
 /// When a tensor operation cannot succeed.
 ///
 pub type TensorError {
+  AxisNotFound
   CannotBroadcast
   IncompatibleAxes
   IncompatibleShape
@@ -3357,8 +3358,8 @@ pub fn in_situ_mean(
 /// The first `Axis` for which the given `find` function returns `True` is
 /// selected for joining.
 ///
-/// If the `find` function returns `False` for every `Axis`, the tensors will
-/// be joined along the first `Axis`.
+/// If the `find` function returns `False` for every `Axis`, an `AxisNotFound`
+/// error is returned.
 ///
 /// ## Examples
 ///
@@ -3390,6 +3391,9 @@ pub fn in_situ_mean(
 /// )
 /// Nil
 ///
+/// > concat([a, b], with: fn(_) { False })
+/// Error(AxisNotFound)
+///
 /// > concat([a, b], with: fn(a) { axis.name(a) == "X" })
 /// Error(IncompatibleShape)
 ///
@@ -3419,13 +3423,7 @@ pub fn concat(
     |> iterator.index
     |> iterator.find(one_that: fn(item) { find(item.1) })
     |> result.map(with: fn(x) { x.0 })
-    |> result.lazy_or(fn() {
-      case new_axes {
-        [_, ..] -> Ok(0)
-        _else -> Error(Nil)
-      }
-    })
-    |> result.replace_error(IncompatibleShape),
+    |> result.replace_error(AxisNotFound),
   )
   use new_axes <- result.try({
     use new_axes, x <- list.try_fold(over: rest, from: new_axes)
diff --git a/test/argamak/tensor_test.gleam b/test/argamak/tensor_test.gleam
index 8cd79cf..a4c2c48 100644
--- a/test/argamak/tensor_test.gleam
+++ b/test/argamak/tensor_test.gleam
@@ -2003,7 +2003,7 @@ pub fn concat_test() {
   let assert Ok(x) =
     x
     |> list.repeat(times: 3)
-    |> tensor.concat(with: fn(_) { False })
+    |> tensor.concat(with: fn(_) { True })
   x
   |> should_share_native_format
   |> tensor.axes
@@ -2026,6 +2026,10 @@ pub fn concat_test() {
   |> tensor.to_floats
   |> should.equal([0.0, 1.0, 4.0, 2.0, 3.0, 5.0])
 
+  [a, b]
+  |> tensor.concat(with: fn(_) { False })
+  |> should.equal(Error(tensor.AxisNotFound))
+
   let error = Error(tensor.IncompatibleShape)
 
   [a, b]