Reject non-None values when encoding with explicit ValueType.NULL

encode_value() with an explicit ValueType.NULL silently discarded non-None
values, returning 8 zero bytes. Since value=None is already caught at the top
of the function, reaching the NULL branch with a non-None value is always a
caller bug. Now raises EncodeError instead of silently losing data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/types.py

@@ -215,7 +215,12 @@ def encode_value(value: Any, value_type: ValueType | None = None) -> tuple[bytes
             raise EncodeError(f"Expected bytes for BLOB, got {type(value).__name__}")
         return encode_blob(bytes(value)), value_type
     elif value_type == ValueType.NULL:
-        return b"\x00" * 8, value_type
+        # value is None is already handled at line 157-158 above, so reaching
+        # here means value is not None with explicit NULL type — always a bug.
+        raise EncodeError(
+            f"Cannot encode non-None value {value!r} as NULL. "
+            f"Pass value=None or use the appropriate ValueType."
+        )
     else:
         raise EncodeError(f"Unknown value type: {value_type}")
 

tests/test_types.py

@@ -682,3 +682,12 @@ def test_decode_null_truncated(self) -> None:
     def test_decode_null_empty(self) -> None:
         with pytest.raises(DecodeError, match="8 bytes"):
             decode_value(b"", ValueType.NULL)
+
+    def test_encode_value_null_type_with_non_none_raises(self) -> None:
+        """Explicit ValueType.NULL with a non-None value should raise EncodeError."""
+        with pytest.raises(EncodeError, match="Cannot encode non-None value"):
+            encode_value(42, ValueType.NULL)
+
+    def test_encode_value_null_type_with_string_raises(self) -> None:
+        with pytest.raises(EncodeError, match="Cannot encode non-None value"):
+            encode_value("hello", ValueType.NULL)