diff --git a/lib/mox.ex b/lib/mox.ex
index a69fa1e..ab184e7 100644
--- a/lib/mox.ex
+++ b/lib/mox.ex
@@ -523,7 +523,8 @@ defmodule Mox do
       end
 
     for behaviour <- behaviours,
-        {fun, arity} <- behaviour.behaviour_info(:callbacks) do
+        {fun, arity} <- behaviour.behaviour_info(:callbacks),
+        function_exported?(mock, fun, arity) do
       stub(mock, fun, :erlang.make_fun(module, fun, arity))
     end
 
diff --git a/test/mox_test.exs b/test/mox_test.exs
index 8b1742a..cb9aa76 100644
--- a/test/mox_test.exs
+++ b/test/mox_test.exs
@@ -583,6 +583,16 @@ defmodule MoxTest do
       def exponent(x, y), do: :math.pow(x, y)
     end
 
+    defmodule SciCalcFullImplementation do
+      @behaviour Calculator
+      def add(x, y), do: x + y
+      def mult(x, y), do: x * y
+
+      @behaviour ScientificCalculator
+      def exponent(x, y), do: :math.pow(x, y)
+      def sin(x), do: :math.sin(x)
+    end
+
     test "can override stubs" do
       in_all_modes(fn ->
         stub_with(CalcMock, CalcImplementation)
@@ -603,6 +613,29 @@ defmodule MoxTest do
       end)
     end
 
+    test "stubs functions which are optional callbacks" do
+      in_all_modes(fn ->
+        stub_with(SciCalcMock, SciCalcFullImplementation)
+        assert SciCalcMock.add(1, 2) == 3
+        assert SciCalcMock.mult(3, 4) == 12
+        assert SciCalcMock.exponent(2, 10) == 1024
+        assert SciCalcMock.sin(0) == 0.0
+      end)
+    end
+
+    test "skips undefined functions which are optional callbacks" do
+      in_all_modes(fn ->
+        stub_with(SciCalcMockWithoutOptional, SciCalcImplementation)
+        assert SciCalcMockWithoutOptional.add(1, 2) == 3
+        assert SciCalcMockWithoutOptional.mult(3, 4) == 12
+        assert SciCalcMockWithoutOptional.exponent(2, 10) == 1024
+
+        assert_raise UndefinedFunctionError, fn ->
+          SciCalcMockWithoutOptional.sin(1)
+        end
+      end)
+    end
+
     test "Leaves behaviours not implemented by the module un-stubbed" do
       in_all_modes(fn ->
         stub_with(SciCalcMock, CalcImplementation)
diff --git a/test/support/mocks.ex b/test/support/mocks.ex
index 3083342..d855695 100644
--- a/test/support/mocks.ex
+++ b/test/support/mocks.ex
@@ -1,5 +1,6 @@
 Mox.defmock(CalcMock, for: Calculator)
 Mox.defmock(SciCalcMock, for: [Calculator, ScientificCalculator])
+Mox.defmock(SciCalcMockWithoutOptional, for: [Calculator, ScientificCalculator], skip_optional_callbacks: true)
 
 Mox.defmock(MyMockWithoutModuledoc, for: Calculator)
 Mox.defmock(MyMockWithFalseModuledoc, for: Calculator, moduledoc: false)