diff --git a/README.md b/README.md
index 1b4272b..e1f8477 100644
--- a/README.md
+++ b/README.md
@@ -289,6 +289,33 @@ schema = {
 - strip-whitespaces
 - left-strip
 
+##### map
+
+Maps a raw value to another one. By default, a value which is not one of the
+`values` keys raises an error. Set the optional `default` parameter to map those
+values instead of failing:
+
+```python
+{
+    "key": "unit-of-measurement",
+    "column-number": 12,
+    "type": "int",
+    "pre-processors": [
+        {
+            "name": "map",
+            "parameters": {"values": {"K": 0, "A": 1, "L": 2}, "default": 0},
+        }
+    ],
+}
+```
+
+An empty string can be used as a key to map blank values:
+`{"values": {"": 0, "K": 1}}`. Note that a field whose raw value is empty is
+never given to its pre-processors: it fails as a required field, or is parsed as
+`None` when the field is declared `"optional": True`. Mapping an empty value
+therefore only applies when a previous pre-processor produces an empty string,
+for instance `strip-whitespaces` on a blank columnar field.
+
 #### Validators
 
 - regex-matches
diff --git a/magicparse/pre_processors.py b/magicparse/pre_processors.py
index 2026f65..c7c4c82 100644
--- a/magicparse/pre_processors.py
+++ b/magicparse/pre_processors.py
@@ -38,16 +38,27 @@ def key() -> str:
         return "left-pad-zeroes"
 
 
+class _NoDefault:
+    "Sentinel allowing 'None' to be used as a mapping default value"
+
+
+NO_DEFAULT = _NoDefault()
+
+
 class Map(PreProcessor):
-    def __init__(self, on_error: OnError, values: dict[str, Any]) -> None:
+    def __init__(self, on_error: OnError, values: dict[str, Any], default: Any = NO_DEFAULT) -> None:
         super().__init__(on_error)
         self.values = values
+        self.default = default
         self._keys = ", ".join(f"'{key}'" for key in self.values.keys())
 
     def apply(self, value: str) -> str:
         try:
             return self.values[value]
         except:
+            if not isinstance(self.default, _NoDefault):
+                return self.default
+
             raise ValueError(f"value '{value}' does not map to any values in [{self._keys}]")
 
     @staticmethod
diff --git a/tests/test_pre_processors.py b/tests/test_pre_processors.py
index a4413e2..a771e09 100644
--- a/tests/test_pre_processors.py
+++ b/tests/test_pre_processors.py
@@ -1,6 +1,7 @@
 import re
 from typing import Any
 from magicparse.pre_processors import (
+    NO_DEFAULT,
     LeftPadZeroes,
     Map,
     PreProcessor,
@@ -23,6 +24,15 @@ def test_map(self):
         pre_processor = PreProcessor.build({"name": "map", "parameters": {"values": {"input": "output"}}})
         assert isinstance(pre_processor, Map)
         assert pre_processor.values == {"input": "output"}
+        assert pre_processor.default is NO_DEFAULT
+
+    def test_map_with_default(self):
+        pre_processor = PreProcessor.build(
+            {"name": "map", "parameters": {"values": {"input": "output"}, "default": "fallback"}}
+        )
+        assert isinstance(pre_processor, Map)
+        assert pre_processor.values == {"input": "output"}
+        assert pre_processor.default == "fallback"
 
     def test_replace(self):
         pre_processor = PreProcessor.build({"name": "replace", "parameters": {"pattern": "aa", "replacement": "bb"}})
@@ -81,6 +91,26 @@ def test_known_input(self):
         pre_processor = PreProcessor.build({"name": "map", "parameters": {"values": {"A": "1", "B": "2"}}})
         assert pre_processor.apply("A") == "1"
 
+    def test_unknown_input_with_default(self):
+        pre_processor = PreProcessor.build(
+            {"name": "map", "parameters": {"values": {"A": "1", "B": "2"}, "default": "0"}}
+        )
+        assert pre_processor.apply("an input") == "0"
+
+    def test_known_input_with_default(self):
+        pre_processor = PreProcessor.build(
+            {"name": "map", "parameters": {"values": {"A": "1", "B": "2"}, "default": "0"}}
+        )
+        assert pre_processor.apply("A") == "1"
+
+    def test_none_default(self):
+        pre_processor = PreProcessor.build({"name": "map", "parameters": {"values": {"A": "1"}, "default": None}})
+        assert pre_processor.apply("an input") is None
+
+    def test_empty_value_can_be_mapped(self):
+        pre_processor = PreProcessor.build({"name": "map", "parameters": {"values": {"": "1", "A": "2"}}})
+        assert pre_processor.apply("") == "1"
+
 
 class TestReplace(TestCase):
     def test_pattern_not_found(self):