diff --git a/narwhals/functions.py b/narwhals/functions.py
index b380aec7b..4e31d5b41 100644
--- a/narwhals/functions.py
+++ b/narwhals/functions.py
@@ -62,7 +62,7 @@ def concat(
         A new DataFrame, Lazyframe resulting from the concatenation.
 
     Raises:
-        NotImplementedError: The items to concatenate should either all be eager, or all lazy
+        TypeError: The items to concatenate should either all be eager, or all lazy
 
     Examples:
         Let's take an example of vertical concatenation:
diff --git a/narwhals/stable/v1/__init__.py b/narwhals/stable/v1/__init__.py
index 80b3cc753..52d75678f 100644
--- a/narwhals/stable/v1/__init__.py
+++ b/narwhals/stable/v1/__init__.py
@@ -2758,7 +2758,7 @@ def concat(
         A new DataFrame, Lazyframe resulting from the concatenation.
 
     Raises:
-        NotImplementedError: The items to concatenate should either all be eager, or all lazy
+        TypeError: The items to concatenate should either all be eager, or all lazy
 
     Examples:
         Let's take an example of vertical concatenation:
diff --git a/narwhals/utils.py b/narwhals/utils.py
index a639f4162..817c2bc9a 100644
--- a/narwhals/utils.py
+++ b/narwhals/utils.py
@@ -194,8 +194,8 @@ def validate_laziness(items: Iterable[Any]) -> None:
         all(isinstance(item, LazyFrame) for item in items)
     ):
         return
-    msg = "The items to concatenate should either all be eager, or all lazy"
-    raise NotImplementedError(msg)
+    msg = f"The items to concatenate should either all be eager, or all lazy, got: {[type(item) for item in items]}"
+    raise TypeError(msg)
 
 
 def maybe_align_index(
diff --git a/tests/frame/invalid_test.py b/tests/frame/invalid_test.py
index 9abf4bd2c..257dade28 100644
--- a/tests/frame/invalid_test.py
+++ b/tests/frame/invalid_test.py
@@ -37,7 +37,7 @@ def test_native_vs_non_native() -> None:
 def test_validate_laziness() -> None:
     df = pl.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
     with pytest.raises(
-        NotImplementedError,
+        TypeError,
         match=("The items to concatenate should either all be eager, or all lazy"),
     ):
         nw.concat([nw.from_native(df, eager_only=True), nw.from_native(df).lazy()])  # type: ignore[list-item]