diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 97b24e7..15d6bc9 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -32,3 +32,10 @@ repos:
       args: [ --fix, --preview ]
     # Run the formatter.
     - id: ruff-format
+- repo: local
+  hooks:
+    - id: mypy
+      name: Run the mypy type linter
+      language: system
+      types: [python]
+      entry: 'make mypy'
diff --git a/src/ttotp/__main__.py b/src/ttotp/__main__.py
index 9f0e0c4..5bf80f8 100755
--- a/src/ttotp/__main__.py
+++ b/src/ttotp/__main__.py
@@ -244,8 +244,8 @@ class TOTPData:
         )
 
 
-def search_preprocess(s):
-    def replace_escape_sequence(m):
+def search_preprocess(s: str) -> str:
+    def replace_escape_sequence(m: re.Match[str]) -> str:
         s = m.group(0)
         if s == "\\ ":
             return " "