diff --git a/adtl/__init__.py b/adtl/__init__.py
index 7783079..3df939a 100644
--- a/adtl/__init__.py
+++ b/adtl/__init__.py
@@ -134,6 +134,10 @@ def get_value_unhashed(row: StrDict, rule: Rule, ctx: Context = None) -> Any:
                 value = rule["values"].get(value, value)
             else:
                 value = rule["values"].get(value)
+
+            # recheck if value is empty after mapping (use to map values to None)
+            if value == "":
+                return None
         # Either source_unit / unit OR source_date / date triggers conversion
         # do not parse units if value is empty
         if "source_unit" in rule and "unit" in rule:
@@ -1055,6 +1059,10 @@ def main(argv=None):
     include_defs = args.include_def or []
     spec = Parser(args.spec, include_defs=include_defs, quiet=args.quiet)
 
+    # check for incompatible options
+    if spec.header.get("returnUnmatched") and args.parquet:
+        raise ValueError("returnUnmatched and parquet options are incompatible")
+
     # run adtl
     adtl_output = spec.parse(args.file, encoding=args.encoding)
     adtl_output.save(args.output or spec.name, args.parquet)
diff --git a/docs/specification.md b/docs/specification.md
index 716cf80..26ddcfd 100644
--- a/docs/specification.md
+++ b/docs/specification.md
@@ -58,8 +58,8 @@ if not present in a datafile, following the same syntax as `fieldPattern` key.
   terminal describing the error in the transformation. Transformations requiring multiple
   parameters will only return the current field value that was not transformed.
   > :warning: This is likely to return columns with non-matching datatypes. External json
-  validation may fail, as will attempting to use the `--parquet` option to save outputs as
-  parquet files (which required a consistent type down each column).
+  validation may fail. This option is incompatible with the `--parquet` option to save
+  outputs as parquet files (which required a consistent type down each column).
 
 ## Validation
 
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 9640e02..da6fe3b 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -1227,6 +1227,23 @@ def test_main_parquet():
     Path("output-table.parquet").unlink()
 
 
+def test_main_parquet_error():
+
+    ARG = [
+        str(TEST_PARSERS_PATH / "return-unmapped.toml"),
+        str(TEST_SOURCES_PATH / "return-unmapped.csv"),
+        "-o",
+        "output",
+        "--encoding",
+        "utf-8",
+    ]
+
+    with pytest.raises(
+        ValueError, match="returnUnmatched and parquet options are incompatible"
+    ):
+        parser.main(ARG + ["--parquet"])
+
+
 @responses.activate
 def test_main_web_schema(snapshot):
     # test with schema on the web