diff --git a/py/defs.bzl b/py/defs.bzl
index 89508537..083329d5 100644
--- a/py/defs.bzl
+++ b/py/defs.bzl
@@ -22,12 +22,12 @@ py_unpacked_wheel = _py_unpacked_wheel
 resolutions = _resolutions
 
 def _py_binary_or_test(name, rule, srcs, main, deps = [], resolutions = {}, **kwargs):
-    if type(main) not in ["string", "Label"]:
+    if main and type(main) not in ["string", "Label"]:
         fail("main must be a Label or a string, not {}".format(type(main)))
 
     # Compatibility with rules_python, see docs in py_executable.bzl
     main_target = "_{}.find_main".format(name)
-    if type(main) == "string":
+    if type(main) != "Label":
         determine_main(
             name = main_target,
             target_name = name,