diff --git a/lib/rdf/datatypes/boolean.ex b/lib/rdf/datatypes/boolean.ex
index 34b79c5..0b150c5 100644
--- a/lib/rdf/datatypes/boolean.ex
+++ b/lib/rdf/datatypes/boolean.ex
@@ -54,6 +54,8 @@ defmodule RDF.Boolean do
     end
   end
 
+  def cast(_), do: nil
+
 
   @doc """
   Returns `RDF.true` if the effective boolean value of the given argument is `RDF.false`, or `RDF.false` if it is `RDF.true`.
diff --git a/lib/rdf/datatypes/date.ex b/lib/rdf/datatypes/date.ex
index 7032a01..7fcc07c 100644
--- a/lib/rdf/datatypes/date.ex
+++ b/lib/rdf/datatypes/date.ex
@@ -86,4 +86,6 @@ defmodule RDF.Date do
     end
   end
 
+  def cast(_), do: nil
+
 end
diff --git a/lib/rdf/datatypes/date_time.ex b/lib/rdf/datatypes/date_time.ex
index 31cd3ad..c3aab50 100644
--- a/lib/rdf/datatypes/date_time.ex
+++ b/lib/rdf/datatypes/date_time.ex
@@ -88,6 +88,8 @@ defmodule RDF.DateTime do
     end
   end
 
+  def cast(_), do: nil
+
 
   def tz(%Literal{value: %NaiveDateTime{}}), do: ""
 
diff --git a/lib/rdf/datatypes/decimal.ex b/lib/rdf/datatypes/decimal.ex
index efeac0f..22f9ca3 100644
--- a/lib/rdf/datatypes/decimal.ex
+++ b/lib/rdf/datatypes/decimal.ex
@@ -96,6 +96,8 @@ defmodule RDF.Decimal do
     end
   end
 
+  def cast(_), do: nil
+
 
   def equal_value?(left, right), do: RDF.Numeric.equal_value?(left, right)
 
diff --git a/lib/rdf/datatypes/double.ex b/lib/rdf/datatypes/double.ex
index 12a50a6..516a889 100644
--- a/lib/rdf/datatypes/double.ex
+++ b/lib/rdf/datatypes/double.ex
@@ -128,6 +128,8 @@ defmodule RDF.Double do
     end
   end
 
+  def cast(_), do: nil
+
 
   def equal_value?(left, right), do: RDF.Numeric.equal_value?(left, right)
 
diff --git a/lib/rdf/datatypes/integer.ex b/lib/rdf/datatypes/integer.ex
index 7508465..e5bb870 100644
--- a/lib/rdf/datatypes/integer.ex
+++ b/lib/rdf/datatypes/integer.ex
@@ -58,6 +58,8 @@ defmodule RDF.Integer do
     end
   end
 
+  def cast(_), do: nil
+
 
   def equal_value?(left, right), do: RDF.Numeric.equal_value?(left, right)
 
diff --git a/lib/rdf/datatypes/lang_string.ex b/lib/rdf/datatypes/lang_string.ex
index d8e3fd3..afd907e 100644
--- a/lib/rdf/datatypes/lang_string.ex
+++ b/lib/rdf/datatypes/lang_string.ex
@@ -53,8 +53,9 @@ defmodule RDF.LangString do
     end
   end
 
+
   def cast(_) do
-    raise "not supported"
+    nil
   end
 
 end
diff --git a/lib/rdf/datatypes/string.ex b/lib/rdf/datatypes/string.ex
index 9abdd96..3d21400 100644
--- a/lib/rdf/datatypes/string.ex
+++ b/lib/rdf/datatypes/string.ex
@@ -91,4 +91,6 @@ defmodule RDF.String do
     end
   end
 
+  def cast(_), do: nil
+
 end
diff --git a/lib/rdf/datatypes/time.ex b/lib/rdf/datatypes/time.ex
index b28f640..2d6e8e0 100644
--- a/lib/rdf/datatypes/time.ex
+++ b/lib/rdf/datatypes/time.ex
@@ -141,4 +141,6 @@ defmodule RDF.Time do
     end
   end
 
+  def cast(_), do: nil
+
 end
diff --git a/test/unit/datatypes/boolean_test.exs b/test/unit/datatypes/boolean_test.exs
index 355d8fa..458ed4d 100644
--- a/test/unit/datatypes/boolean_test.exs
+++ b/test/unit/datatypes/boolean_test.exs
@@ -112,6 +112,10 @@ defmodule RDF.BooleanTest do
     test "with literals of unsupported datatypes" do
       assert RDF.DateTime.now() |> RDF.Boolean.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Boolean.cast(:foo) == nil
+    end
   end
 
 
diff --git a/test/unit/datatypes/date_test.exs b/test/unit/datatypes/date_test.exs
index 7e761f0..52a85ff 100644
--- a/test/unit/datatypes/date_test.exs
+++ b/test/unit/datatypes/date_test.exs
@@ -109,6 +109,10 @@ defmodule RDF.DateTest do
       assert RDF.integer(1) |> RDF.Date.cast() == nil
       assert RDF.decimal(3.14) |> RDF.Date.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Date.cast(:foo) == nil
+    end
   end
 
 end
diff --git a/test/unit/datatypes/date_time_test.exs b/test/unit/datatypes/date_time_test.exs
index d6ff8dd..d971b8f 100644
--- a/test/unit/datatypes/date_time_test.exs
+++ b/test/unit/datatypes/date_time_test.exs
@@ -163,6 +163,10 @@ defmodule RDF.DateTimeTest do
       assert RDF.integer(1)    |> RDF.DateTime.cast() == nil
       assert RDF.decimal(3.14) |> RDF.DateTime.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.DateTime.cast(:foo) == nil
+    end
   end
 
 
diff --git a/test/unit/datatypes/decimal_test.exs b/test/unit/datatypes/decimal_test.exs
index cfce14f..70ac805 100644
--- a/test/unit/datatypes/decimal_test.exs
+++ b/test/unit/datatypes/decimal_test.exs
@@ -117,6 +117,10 @@ defmodule RDF.DecimalTest do
     test "with literals of unsupported datatypes" do
       assert RDF.DateTime.now() |> RDF.Decimal.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Decimal.cast(:foo) == nil
+    end
   end
 
 
diff --git a/test/unit/datatypes/double_test.exs b/test/unit/datatypes/double_test.exs
index cf0910b..b5e8f76 100644
--- a/test/unit/datatypes/double_test.exs
+++ b/test/unit/datatypes/double_test.exs
@@ -105,6 +105,10 @@ defmodule RDF.DoubleTest do
     test "with literals of unsupported datatypes" do
       assert RDF.DateTime.now() |> RDF.Double.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Double.cast(:foo) == nil
+    end
   end
 
 end
diff --git a/test/unit/datatypes/integer_test.exs b/test/unit/datatypes/integer_test.exs
index a90a99b..c20dde7 100644
--- a/test/unit/datatypes/integer_test.exs
+++ b/test/unit/datatypes/integer_test.exs
@@ -70,6 +70,10 @@ defmodule RDF.IntegerTest do
     test "with literals of unsupported datatypes" do
       assert RDF.DateTime.now() |> RDF.Integer.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Integer.cast(:foo) == nil
+    end
   end
 
 
diff --git a/test/unit/datatypes/string_test.exs b/test/unit/datatypes/string_test.exs
index 328152d..4ca853d 100644
--- a/test/unit/datatypes/string_test.exs
+++ b/test/unit/datatypes/string_test.exs
@@ -122,6 +122,10 @@ defmodule RDF.StringTest do
       assert RDF.decimal("NAN") |> RDF.String.cast() == nil
       assert RDF.double(true)   |> RDF.String.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.String.cast(:foo) == nil
+    end
   end
 
 end
diff --git a/test/unit/datatypes/time_test.exs b/test/unit/datatypes/time_test.exs
index 088e7d3..ca6516b 100644
--- a/test/unit/datatypes/time_test.exs
+++ b/test/unit/datatypes/time_test.exs
@@ -96,6 +96,10 @@ defmodule RDF.TimeTest do
       assert RDF.integer(1) |> RDF.Time.cast() == nil
       assert RDF.decimal(3.14) |> RDF.Time.cast() == nil
     end
+
+    test "with non-RDF terms" do
+      assert RDF.Time.cast(:foo) == nil
+    end
   end
 
 end