diff --git a/stubdefaulter.py b/stubdefaulter.py
index e4f3c0e..030549b 100644
--- a/stubdefaulter.py
+++ b/stubdefaulter.py
@@ -69,6 +69,14 @@ def leave_Param(
         return updated_node
 
 
+def get_end_lineno(node: ast.FunctionDef | ast.AsyncFunctionDef) -> int:
+    if sys.version_info >= (3, 8):
+        assert node.end_lineno is not None
+        return node.end_lineno
+    else:
+        return max(child.lineno for child in ast.iter_child_nodes(node))
+
+
 def replace_defaults_in_func(
     stub_lines: list[str],
     node: ast.FunctionDef | ast.AsyncFunctionDef,
@@ -78,8 +86,8 @@ def replace_defaults_in_func(
         sig = inspect.signature(runtime_func)
     except Exception:
         return {}
-    assert node.end_lineno is not None
-    lines = stub_lines[node.lineno - 1 : node.end_lineno]
+    end_lineno = get_end_lineno(node)
+    lines = stub_lines[node.lineno - 1 : end_lineno]
     indentation = len(lines[0]) - len(lines[0].lstrip())
     cst = libcst.parse_statement(
         textwrap.dedent("".join(line + "\n" for line in lines))
@@ -88,7 +96,7 @@ def replace_defaults_in_func(
     assert isinstance(modified, libcst.FunctionDef)
     new_code = textwrap.indent(libcst.Module(body=[modified]).code, " " * indentation)
     output_dict = {node.lineno - 1: new_code.splitlines()}
-    for i in range(node.lineno, node.end_lineno):
+    for i in range(node.lineno, end_lineno):
         output_dict[i] = []
     return output_dict