diff --git a/lib/rdf/prefix_map.ex b/lib/rdf/prefix_map.ex
index c8b7f2e..b672583 100644
--- a/lib/rdf/prefix_map.ex
+++ b/lib/rdf/prefix_map.ex
@@ -38,10 +38,20 @@ defmodule RDF.PrefixMap do
     do: normalize({String.to_atom(prefix), namespace})
 
   defp normalize({prefix, namespace}) when is_binary(namespace),
-       do: normalize({prefix, IRI.new(namespace)})
+    do: normalize({prefix, IRI.new(namespace)})
 
-  defp normalize({prefix, _}),
-    do: raise("Invalid prefix on PrefixMap: #{inspect(prefix)}}")
+  defp normalize({prefix, namespace}) when is_atom(namespace) do
+    if function_exported?(namespace, :__base_iri__, 0) do
+      normalize({prefix, apply(namespace, :__base_iri__, [])})
+    else
+      raise ArgumentError,
+            "Invalid prefix mapping for #{inspect(prefix)}, #{inspect(namespace)} is not a vocabulary namespace"
+    end
+  end
+
+  defp normalize({prefix, namespace}),
+    do:
+      raise(ArgumentError, "Invalid prefix mapping: #{inspect(prefix)} => #{inspect(namespace)}")
 
   @doc """
   Adds a prefix mapping the given `RDF.PrefixMap`.
@@ -92,7 +102,8 @@ defmodule RDF.PrefixMap do
       {:ok, %__MODULE__{map: Map.merge(map1, map2)}}
     else
       conflicts ->
-        {:error, "conflicting prefix mappings: #{conflicts |> Stream.map(&inspect/1) |> Enum.join(", ")}"}
+        {:error,
+         "conflicting prefix mappings: #{conflicts |> Stream.map(&inspect/1) |> Enum.join(", ")}"}
     end
   end
 
diff --git a/test/unit/prefix_map_test.exs b/test/unit/prefix_map_test.exs
index 872cab0..0b67dcf 100644
--- a/test/unit/prefix_map_test.exs
+++ b/test/unit/prefix_map_test.exs
@@ -20,6 +20,11 @@ defmodule RDF.PrefixMapTest do
     ex3: @ex_ns3
   }}
 
+  @example4 %PrefixMap{map: %{
+    ex1: @ex_ns1,
+    ex: RDF.iri(EX.__base_iri__)
+  }}
+
   test "new/0" do
     assert PrefixMap.new() == %PrefixMap{}
   end
@@ -38,6 +43,13 @@ defmodule RDF.PrefixMapTest do
                ex2: "http://example.com/bar#"
              ) == @example2
     end
+
+    test "when the IRI namespace is given as a RDF.Vocabulary.Namespace" do
+      assert PrefixMap.new(
+               ex1: "http://example.com/foo/",
+               ex: EX
+             ) == @example4
+    end
   end
 
   describe "add/3" do
@@ -45,14 +57,24 @@ defmodule RDF.PrefixMapTest do
       assert PrefixMap.add(@example1, :ex2, @ex_ns2) == {:ok, @example2}
     end
 
-    test "with the prefix is given as a string" do
+    test "when the prefix is given as a string" do
       assert PrefixMap.add(@example1, "ex2", @ex_ns2) == {:ok, @example2}
     end
 
-    test "with the IRI namespace is given as a string" do
+    test "when the IRI namespace is given as a string" do
       assert PrefixMap.add(@example1, :ex2, "http://example.com/bar#") == {:ok, @example2}
     end
 
+    test "when the IRI namespace is given as a RDF.Vocabulary.Namespace" do
+      assert PrefixMap.add(@example1, :ex, EX) == {:ok, @example4}
+    end
+
+    test "when the IRI namespace is given as an atom" do
+      assert_raise ArgumentError, "Invalid prefix mapping for :ex, :foo is not a vocabulary namespace", fn ->
+        PrefixMap.add(@example1, :ex, :foo)
+      end
+    end
+
     test "when a mapping of the given prefix to the same namespace already exists" do
       assert PrefixMap.add(@example2, :ex2, "http://example.com/bar#") == {:ok, @example2}
     end