diff --git a/src/copapy/__init__.py b/src/copapy/__init__.py
index db79e08..0ed8e17 100644
--- a/src/copapy/__init__.py
+++ b/src/copapy/__init__.py
@@ -396,8 +396,12 @@ def compile_to_instruction_list(node_list: Iterable[Node], sdb: stencil_database
     dw.write_com(binw.Command.COPY_CODE)
     dw.write_int(0)
     dw.write_int(aux_function_lengths)
+    idat: int = 0
     for name, _, _ in aux_function_mem_layout:
-        dw.write_bytes(sdb.get_function_code(name))
+        dat = sdb.get_function_code(name)
+        dw.write_bytes(dat)
+        idat += len(dat)
+    assert idat == aux_function_lengths
 
     # Prepare program code and relocations
     object_addr_lookup = {net: offs for net, offs, _ in variable_mem_layout}