diff --git a/beets/library/exceptions.py b/beets/library/exceptions.py
index 39725a4e4..df095aeb8 100644
--- a/beets/library/exceptions.py
+++ b/beets/library/exceptions.py
@@ -8,7 +8,7 @@ class FileOperationError(Exception):
     error, and an unhandled Mutagen exception.
     """
 
-    def __init__(self, path, reason):
+    def __init__(self, path: bytes, reason: Exception) -> None:
         """Create an exception describing an operation on the file at
         `path` with the underlying (chained) exception `reason`.
         """
@@ -16,7 +16,7 @@ class FileOperationError(Exception):
         self.path = path
         self.reason = reason
 
-    def __str__(self):
+    def __str__(self) -> str:
         """Get a string representing the error.
 
         Describe both the underlying reason and the file path in question.
@@ -27,7 +27,7 @@ class FileOperationError(Exception):
 class ReadError(FileOperationError):
     """An error while reading a file (i.e. in `Item.read`)."""
 
-    def __str__(self):
+    def __str__(self) -> str:
         # Formatting super() directly stringifies the proxy object, so call
         # __str__ explicitly to forward to FileOperationError.__str__.
         return f"error reading {super().__str__()}"
@@ -36,7 +36,7 @@ class ReadError(FileOperationError):
 class WriteError(FileOperationError):
     """An error while writing a file (i.e. in `Item.write`)."""
 
-    def __str__(self):
+    def __str__(self) -> str:
         # Formatting super() directly stringifies the proxy object, so call
         # __str__ explicitly to forward to FileOperationError.__str__.
         return f"error writing {super().__str__()}"