diff --git a/devtools/conda-envs/full-environment.yaml b/devtools/conda-envs/full-environment.yaml
index 33abe651..12863c3f 100644
--- a/devtools/conda-envs/full-environment.yaml
+++ b/devtools/conda-envs/full-environment.yaml
@@ -20,4 +20,4 @@ dependencies:
   - pytest
   - codecov
   - pytest-cov
-  - mypy ==0.812
+  - mypy
diff --git a/devtools/conda-envs/min-deps-environment.yaml b/devtools/conda-envs/min-deps-environment.yaml
index ac87bcfd..9d2f4112 100644
--- a/devtools/conda-envs/min-deps-environment.yaml
+++ b/devtools/conda-envs/min-deps-environment.yaml
@@ -11,4 +11,4 @@ dependencies:
   - pytest
   - codecov
   - pytest-cov
-  - mypy ==0.812
+  - mypy
diff --git a/devtools/conda-envs/min-ver-environment.yaml b/devtools/conda-envs/min-ver-environment.yaml
index 0d52d0f6..04c59067 100644
--- a/devtools/conda-envs/min-ver-environment.yaml
+++ b/devtools/conda-envs/min-ver-environment.yaml
@@ -15,4 +15,4 @@ dependencies:
   - pytest
   - codecov
   - pytest-cov
-  - mypy ==0.812
+  - mypy
diff --git a/opt_einsum/contract.py b/opt_einsum/contract.py
index 14ccfaff..0df94d03 100644
--- a/opt_einsum/contract.py
+++ b/opt_einsum/contract.py
@@ -7,7 +7,7 @@
 from typing import Any, Dict, Iterable, List, Optional, Sequence, Tuple
 
 from . import backends, blas, helpers, parser, paths, sharing
-from .typing import ArrayIndexType, ArrayType, Collection, ContractionListType, PathType
+from .typing import ArrayIndexType, ArrayType, ContractionListType, PathType
 
 __all__ = [
     "contract_path",
@@ -75,7 +75,7 @@ def __repr__(self) -> str:
         ]
 
         for n, contraction in enumerate(self.contraction_list):
-            inds, idx_rm, einsum_str, remaining, do_blas = contraction
+            _, _, einsum_str, remaining, do_blas = contraction
 
             if remaining is not None:
                 remaining_str = ",".join(remaining) + "->" + self.output_subscript
@@ -865,7 +865,7 @@ def __str__(self) -> str:
 Shaped = namedtuple("Shaped", ["shape"])
 
 
-def shape_only(shape: Collection[Tuple[int, ...]]) -> Shaped:
+def shape_only(shape: PathType) -> Shaped:
     """Dummy ``numpy.ndarray`` which has a shape only - for generating
     contract expressions.
     """
diff --git a/opt_einsum/parser.py b/opt_einsum/parser.py
index 76b095a2..8e3a00b2 100644
--- a/opt_einsum/parser.py
+++ b/opt_einsum/parser.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-# coding: utf-8
 """
 A functionally equivalent parser of the numpy.einsum input parser
 """
@@ -61,7 +59,7 @@ def has_valid_einsum_chars_only(einsum_str: str) -> bool:
 
 def get_symbol(i: int) -> str:
     """Get the symbol corresponding to int ``i`` - runs through the usual 52
-    letters before resorting to unicode characters, starting at ``chr(192)``.
+    letters before resorting to unicode characters, starting at ``chr(192)`` and skipping surrogates.
 
     **Examples:**
 
@@ -78,7 +76,11 @@ def get_symbol(i: int) -> str:
     """
     if i < 52:
         return _einsum_symbols_base[i]
-    return chr(i + 140)
+    elif i >= 55296:
+        # Skip chr(57343) - chr(55296) as surrogates
+        return chr(i + 2048)
+    else:
+        return chr(i + 140)
 
 
 def gen_unused_symbols(used: str, n: int) -> Iterator[str]:
diff --git a/opt_einsum/paths.py b/opt_einsum/paths.py
index 23a2c49d..eac36301 100644
--- a/opt_einsum/paths.py
+++ b/opt_einsum/paths.py
@@ -352,6 +352,9 @@ def __init__(
         minimize="flops",
         cost_fn="memory-removed",
     ):
+        if (nbranch is not None) and nbranch < 1:
+            raise ValueError(f"The number of branches must be at least one, `nbranch={nbranch}`.")
+
         self.nbranch = nbranch
         self.cutoff_flops_factor = cutoff_flops_factor
         self.minimize = minimize
@@ -827,9 +830,9 @@ def _tree_to_sequence(tree: Tuple[Any, ...]) -> PathType:
             s[0] += (sum(1 for q in t if q < i),)
             t.insert(s[0][-1], i)
 
-        for i in [i for i in j if type(i) != int]:
+        for i_tup in [i_tup for i_tup in j if type(i_tup) != int]:
             s[0] += (len(t) + len(c),)
-            c.append(i)
+            c.append(i_tup)
 
     return s
 
diff --git a/opt_einsum/tests/test_parser.py b/opt_einsum/tests/test_parser.py
new file mode 100644
index 00000000..e81fafdd
--- /dev/null
+++ b/opt_einsum/tests/test_parser.py
@@ -0,0 +1,14 @@
+"""
+Directly tests various parser utility functions.
+"""
+
+from opt_einsum.parser import get_symbol
+
+
+def test_get_symbol():
+    assert get_symbol(2) == "c"
+    assert get_symbol(200000) == "\U00031540"
+    # Ensure we skip surrogates '[\uD800-\uDFFF]'
+    assert get_symbol(55295) == "\ud88b"
+    assert get_symbol(55296) == "\ue000"
+    assert get_symbol(57343) == "\ue7ff"
diff --git a/opt_einsum/tests/test_paths.py b/opt_einsum/tests/test_paths.py
index 03fd1049..c6fef4f5 100644
--- a/opt_einsum/tests/test_paths.py
+++ b/opt_einsum/tests/test_paths.py
@@ -394,6 +394,11 @@ def test_custom_branchbound():
         path, path_info = oe.contract_path(eq, *views, optimize=optimizer)
 
 
+def test_branchbound_validation():
+    with pytest.raises(ValueError):
+        oe.BranchBound(nbranch=0)
+
+
 @pytest.mark.skipif(sys.version_info < (3, 2), reason="requires python3.2 or higher")
 def test_parallel_random_greedy():
     from concurrent.futures import ProcessPoolExecutor
diff --git a/scripts/compare_random_paths.py b/scripts/compare_random_paths.py
index c7917cda..b6d4bf7a 100644
--- a/scripts/compare_random_paths.py
+++ b/scripts/compare_random_paths.py
@@ -33,7 +33,7 @@
 alpha = list("abcdefghijklmnopqrstuvwyxz")
 alpha_dict = {num: x for num, x in enumerate(alpha)}
 
-print("Maximum term size is %d" % (max_size ** max_dims))
+print("Maximum term size is %d" % (max_size**max_dims))
 
 
 def make_term():