diff --git a/src/seriousdb/wal.py b/src/seriousdb/wal.py
index 3acd9df..27a8e94 100644
--- a/src/seriousdb/wal.py
+++ b/src/seriousdb/wal.py
@@ -31,7 +31,7 @@ def from_dict(data: dict) -> "WalEntry":
         Raises
         ------
         TypeError
-            If `data` is not a dictionary.
+            If `data` is not a dictionary, or if key or value is not a string.
         ValueError
             If `data` does not describe a known entry type.
         """
@@ -41,10 +41,24 @@ def from_dict(data: dict) -> "WalEntry":
         op = data.get("op")
         try:
             if op == "set":
-                return SetEntry(key=data["key"], value=data["value"])
+                key = data["key"]
+                value = data["value"]
+
+                if not isinstance(key, str):
+                    raise TypeError("key must be a string")
+                if not isinstance(value, str):
+                    raise TypeError("value must be a string")
+
+                return SetEntry(key=key, value=value)
 
             if op == "delete":
-                return DeleteEntry(key=data["key"])
+                key = data["key"]
+
+                if not isinstance(key, str):
+                    raise TypeError("key must be a string")
+
+                return DeleteEntry(key=key)
+
         except KeyError as e:
             raise ValueError(f"missing required field: {e}") from e
 
diff --git a/tests/test_wal.py b/tests/test_wal.py
index 2cbbf10..7c8a680 100644
--- a/tests/test_wal.py
+++ b/tests/test_wal.py
@@ -69,6 +69,9 @@ def test_replay_drops_and_truncates_a_torn_last_entry(wal, wal_path):
     [
         b'{"op": "set", "key": "b"}\n',  # Missing required field
         b"[]\n",  # Unexpected top-level type
+        b'{"op": "set", "key": null, "value": "b"}\n',  # Key is null
+        b'{"op": "set", "key": 5, "value": "b"}\n',  # Key is not a string
+        b'{"op": "set", "key": "b", "value": [1, 2]}\n',  # Value is not a string
     ],
 )
 def test_replay_recovers_from_malformed_entry(wal, wal_path, malformed):