diff --git a/setup.py b/setup.py
index e77ce33..a08b793 100755
--- a/setup.py
+++ b/setup.py
@@ -221,7 +221,7 @@ def get_ext_module_1(optional):
 )
 
 if custom_arg is None:
-    if impl == "PyPy":
+    if impl == "PyPy" or not c_src_path.exists():
         custom_arg = "py"
     else:
         custom_arg = "py" if pure_py else "c"