diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 951866b..2123c91 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -29,6 +29,5 @@ repos:
     hooks:
       - id: mypy
         additional_dependencies: [types-requests, types-PyYAML]
-        language_version: python3.8
 default_language_version:
   python: python3
diff --git a/src/equation_tree/util/type_check.py b/src/equation_tree/util/type_check.py
index 704db4a..3df5419 100644
--- a/src/equation_tree/util/type_check.py
+++ b/src/equation_tree/util/type_check.py
@@ -22,7 +22,7 @@ def is_numeric(s):
         False
 
     """
-    if isinstance(s, str) and (not s[0].isnumeric() and s[0] != "."):
+    if isinstance(s, str) and (not s[0].isnumeric() and not (s[0] in [".", "-"])):
         return False
     try:
         float(s)