diff --git a/src/cyclebane/graph.py b/src/cyclebane/graph.py
index a2fbfb8..f242c86 100644
--- a/src/cyclebane/graph.py
+++ b/src/cyclebane/graph.py
@@ -195,8 +195,13 @@ def map(
         *,
         value_attr: str = 'value',
     ) -> Graph:
-        """For every value, create a new graph with all successors renamed, merge all
-        resulting graphs."""
+        """
+        Map the graph over the given values by associating source nodes with values.
+
+        All successors of the mapped source nodes are replaced with new nodes, one for
+        each index value. The value is set as an attribute on the new source nodes
+        (but not their successors).
+        """
         root_nodes = tuple(node_values.keys())
         indices = self._get_indices(node_values)
         named = tuple(name for name, _ in indices if name is not None)
@@ -231,7 +236,28 @@ def reduce(
         name: str,
         attrs: None | dict[str, Any] = None,
     ) -> Graph:
-        """Add edges from all nodes (key, index) to new node."""
+        """
+        Reduce over the given index or axis previously created with :py:meth:`map`.
+        `
+
+        If neither index nor axis is given, all axes are reduced.
+
+        Parameters
+        ----------
+        key:
+            The name of the source node to reduce. This is the original name prior to
+            mapping. Note that there is ambiguity if the same was used as 'name' in
+            a previous reduce operation over a subset of indices/axes.
+        index:
+            The name of the index to reduce over. Only one of index and axis can be
+            given.
+        axis:
+            The axis to reduce over. Only one of index and axis can be given.
+        name:
+            The name of the new node.
+        attrs:
+            Attributes to set on the new node.
+        """
         attrs = attrs or {}
         if index is not None and axis is not None:
             raise ValueError('Only one of index and axis can be given')
diff --git a/tests/graph_test.py b/tests/graph_test.py
index 87741e2..6888474 100644
--- a/tests/graph_test.py
+++ b/tests/graph_test.py
@@ -10,6 +10,17 @@
 import cyclebane as cb
 
 
+def test_map_raises_if_mapping_nonexistent_node() -> None:
+    g = nx.DiGraph()
+    g.add_edge('a', 'b')
+
+    graph = cb.Graph(g)
+    with pytest.raises(ValueError):
+        graph.map({'c': [1, 2]})
+    with pytest.raises(ValueError):
+        graph.map({'a': [1, 2], 'c': [1, 2]})
+
+
 def test_map_raises_if_mapping_non_source_node() -> None:
     g = nx.DiGraph()
     g.add_edge('a', 'b')