diff --git a/nutils/evaluable.py b/nutils/evaluable.py
index e8f981e5a..3492d09b8 100644
--- a/nutils/evaluable.py
+++ b/nutils/evaluable.py
@@ -4356,16 +4356,17 @@ def _intbounds_impl(self):
 
 class LoopConcatenate(Loop, Array):
 
-    def __init__(self, func: Array, start: Array, stop: Array, shape: typing.Tuple[Array, ...], index_name: str, length: Array):
+    def __init__(self, func: Array, start: Array, stop: Array, concat_length: Array, index_name: str, length: Array):
         assert isinstance(func, Array), f'func={func}'
         assert _isindex(start), f'start={start}'
         assert _isindex(stop), f'stop={stop}'
-        assert isinstance(shape, tuple) and all(map(_isindex, shape)), f'shape={shape}'
+        assert _isindex(concat_length), f'concat_length={concat_length}'
         self.func = func
         self.start = start
         self.stop = stop
         if not self.func.ndim:
             raise ValueError('expected an array with at least one axis')
+        shape = *func.shape[:-1], concat_length
         super().__init__(init_arg=Tuple(shape), body_arg=Tuple((func, start, stop)), index_name=index_name, length=length, shape=shape, dtype=func.dtype)
 
     def evalf_loop_init(self, shape):
@@ -4942,8 +4943,8 @@ def loop_concatenate(func, index):
     offsets = _SizesToOffsets(chunk_sizes)
     start = Take(offsets, index)
     stop = Take(offsets, index+1)
-    shape = *func.shape[:-1], Take(offsets, index.length)
-    return LoopConcatenate(func, start, stop, shape, index.name, index.length)
+    concat_length = Take(offsets, index.length)
+    return LoopConcatenate(func, start, stop, concat_length, index.name, index.length)
 
 
 @util.shallow_replace