diff --git a/numpy_groupies/tests/test_generic.py b/numpy_groupies/tests/test_generic.py
index 03d7b94..cea8554 100644
--- a/numpy_groupies/tests/test_generic.py
+++ b/numpy_groupies/tests/test_generic.py
@@ -353,6 +353,24 @@ def test_agg_along_axis(aggregate_all, size, func, axis):
     # instead we squeeze out the extra dims in actual.
     np.testing.assert_allclose(actual.squeeze(), expected)
 
+
+def test_custom_callable(aggregate_all):
+    def sum_(array):
+        return array.sum()
+
+    size = (10,)
+    axis = -1
+
+    group_idx = np.zeros(size, dtype=int)
+    array = np.random.randn(*size)
+
+    expected = array.sum(axis=axis, keepdims=True)
+    actual = aggregate_all(group_idx, array, axis=axis, func=sum_, fill_value=0)
+    assert actual.ndim == array.ndim
+
+    np.testing.assert_allclose(actual, expected)
+
+
 def test_argreduction_nD_array_1D_idx(aggregate_all):
     # regression test for GH41
     labels = np.array([0, 0, 2, 2, 2, 1, 1, 2, 2, 1, 1, 0], dtype=int)
diff --git a/numpy_groupies/utils_numpy.py b/numpy_groupies/utils_numpy.py
index fecf9b4..02616c2 100644
--- a/numpy_groupies/utils_numpy.py
+++ b/numpy_groupies/utils_numpy.py
@@ -283,7 +283,7 @@ def input_validation(group_idx, a, size=None, order='C', axis=None,
         else:
             is_form_3 = group_idx.ndim == 1 and a.ndim > 1 and axis is not None
             orig_shape = a.shape if is_form_3 else group_idx.shape
-            if "arg" in func:
+            if isinstance(func, str) and "arg" in func:
                 unravel_shape = orig_shape
             else:
                 unravel_shape = None