diff --git a/lark/lark.py b/lark/lark.py
index 071c9d3..05ad9b1 100644
--- a/lark/lark.py
+++ b/lark/lark.py
@@ -176,6 +176,9 @@ class LarkOptions(Serialize):
 # These option are only used outside of `load_grammar`.
 _LOAD_ALLOWED_OPTIONS = {'postlex', 'transformer', 'use_bytes', 'debug', 'g_regex_flags', 'regex', 'propagate_positions', 'tree_class'}
 
+_VALID_PRIORITY_OPTIONS = ('auto', 'normal', 'invert', None)
+_VALID_AMBIGUITY_OPTIONS = ('auto', 'resolve', 'explicit', 'forest')
+
 
 class Lark(Serialize):
     """Main interface for the library.
@@ -274,9 +277,11 @@ class Lark(Serialize):
         if self.options.priority == 'auto':
             self.options.priority = 'normal'
 
-        assert self.options.priority in ('auto', None, 'normal', 'invert'), 'invalid priority option specified: {}. options are auto, none, normal, invert.'.format(self.options.priority)
+        if self.options.priority not in _VALID_PRIORITY_OPTIONS:
+            raise ValueError("invalid priority option: %r. Must be one of %r" % (self.options.priority, _VALID_PRIORITY_OPTIONS))
         assert self.options.ambiguity not in ('resolve__antiscore_sum', ), 'resolve__antiscore_sum has been replaced with the option priority="invert"'
-        assert self.options.ambiguity in ('resolve', 'explicit', 'forest', 'auto', )
+        if self.options.ambiguity not in _VALID_AMBIGUITY_OPTIONS:
+            raise ValueError("invalid ambiguity option: %r. Must be one of %r" % (self.options.ambiguity, _VALID_AMBIGUITY_OPTIONS))
 
         # Parse the grammar file and compose the grammars (TODO)
         self.grammar = load_grammar(grammar, self.source, re_module, self.options.keep_all_tokens)
diff --git a/tests/test_parser.py b/tests/test_parser.py
index f31ac00..7a0a181 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -1829,6 +1829,20 @@ def _make_parser_test(LEXER, PARSER):
             res = l.parse("a")
             self.assertEqual(res.children[0].data, 'a')
 
+            grammar = """
+            start: a | b
+            a.2: "A"+
+            b.1: "A"+ "B"?
+            """
+
+            l = _Lark(grammar)
+            res = l.parse("AAAA")
+            self.assertEqual(res.children[0].data, 'a')
+
+            l = _Lark(grammar, priority="invert")
+            res = l.parse("AAAA")
+            self.assertEqual(res.children[0].data, 'b')
+
 
 
         @unittest.skipIf(PARSER != 'earley' or LEXER == 'standard', "Currently only Earley supports priority sum in rules")