diff --git a/mistletoe/span_tokenizer.py b/mistletoe/span_tokenizer.py
index 106bcf35..260e6938 100644
--- a/mistletoe/span_tokenizer.py
+++ b/mistletoe/span_tokenizer.py
@@ -33,6 +33,9 @@ def tokenize(content, token_types, fallback_token):
             # there's text between our current position and the next token
             yield fallback_token(content[index:min_index])
         if min_match_obj:                # min_match_obj is not None
-            yield min_token_type(min_match_obj.group(1))
+            yield min_token_type(_first_not_none_group(min_match_obj))
             index += min_match_obj.end() # update pointer
         else: return                     # min_match_obj is None; no more tokens
+
+def _first_not_none_group(match_obj):
+    return next(group for group in match_obj.groups() if group is not None)
diff --git a/test/test_span_token.py b/test/test_span_token.py
index 9c423823..c0cc5b1d 100644
--- a/test/test_span_token.py
+++ b/test/test_span_token.py
@@ -85,6 +85,11 @@ def test_raw(self):
         c = span_token.RawText('some text')
         helpers.check_equal(self, list(t.children)[0], c)
 
+    def test_parse(self):
+        t = span_token.Strong('_some text_')
+        c = span_token.Emphasis('some text')
+        helpers.check_equal(self, list(t.children)[0], c)
+
 class TestInlineCode(unittest.TestCase):
     def test_raw(self):
         t = span_token.InlineCode('some code')