diff --git a/ete4/parser/newick.pyx b/ete4/parser/newick.pyx
index 85281b4d1..a61c79e2c 100644
--- a/ete4/parser/newick.pyx
+++ b/ete4/parser/newick.pyx
@@ -239,6 +239,7 @@ def read_props(str text, long pos, is_leaf, dict parser, check_req=False):
     except (AssertionError, ValueError) as e:
         raise NewickError('parsing %r: %s' % (p0_str, e))
 
+    p1_str = ''  # always defined (like p0_str) so error messages don't fail
     try:
         if pos < len(text) and text[pos] == ':':
             pos = skip_spaces_and_comments(text, pos+1)
diff --git a/tests/test_tree.py b/tests/test_tree.py
index 74d45c68b..078a9c4be 100644
--- a/tests/test_tree.py
+++ b/tests/test_tree.py
@@ -345,6 +345,12 @@ def test_newick_formats(self):
         # unsupported newick stream
         self.assertRaises(Exception, Tree, [1,2,3])
 
+    def test_newick_missing_required_value(self):
+        # A strict parser with a required second field (dist/support) must
+        # raise a clean NewickError when that value is missing, not crash with
+        # UnboundLocalError from the error handler (issue #799).
+        self.assertRaises(NewickError, Tree, '(A);', parser=3)
+
     def test_newick_multisupport(self):
         nw = '((a,b)2/3:4,(c,d)5/6:7);'
         t = Tree(nw, parser='multisupport')