diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 5bc7bcb..6b1df3b 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -29,6 +29,7 @@
 import collections
 import json
 import logging as log
+import operator
 import os
 import re
 import sys
@@ -619,11 +620,19 @@
   def _has_arg(self, arg):
     return arg in self.target.args
 
+  # Whether an arg has multiple occurences (see argparse action='append')
+  def _is_append_arg(self, arg):
+    return operator.countOf(arg, self.target.args) > 1
+
   def _has_arg_value(self, arg):
+    # TODO: we'll probably need a set of helper functions to deal with append
+    # args as well.
+    assert(not self._is_append_arg(arg))
     i = self.target.args.index(arg)
     return not self.target.args[i + 1].startswith('--')
 
   def _get_arg_value(self, arg):
+    assert(self._has_arg_value(arg))
     i = self.target.args.index(arg)
     return self.target.args[i + 1]
 
@@ -633,6 +642,7 @@
     self.target.args[i + 1] = value
 
   def _delete_arg(self, arg):
+    assert(not self._is_append_arg(arg))
     hasValue = self._has_arg_value(arg)
     i = self.target.index(arg)
     self.target.args.pop(i)