diff --git a/lib/validates_timeliness/conversion.rb b/lib/validates_timeliness/conversion.rb
index af9ffb6..81d8468 100644
--- a/lib/validates_timeliness/conversion.rb
+++ b/lib/validates_timeliness/conversion.rb
@@ -2,7 +2,7 @@ module ValidatesTimeliness
   module Conversion
 
     def type_cast_value(value, type)
-      return nil if value.nil?
+      return nil if value.nil? || !value.respond_to?(:to_time)
 
       value = value.in_time_zone if value.acts_like?(:time) && @timezone_aware
       value = case type
diff --git a/spec/validates_timeliness/conversion_spec.rb b/spec/validates_timeliness/conversion_spec.rb
index 25b6af4..656ff57 100644
--- a/spec/validates_timeliness/conversion_spec.rb
+++ b/spec/validates_timeliness/conversion_spec.rb
@@ -22,6 +22,10 @@ describe ValidatesTimeliness::Conversion do
       it "should return date part of datetime value" do
         type_cast_value(DateTime.new(2010, 1, 1, 0, 0, 0), :date).should == Date.new(2010, 1, 1)
       end
+
+      it 'should return nil for invalid value types' do
+        type_cast_value(12, :date).should == nil
+      end
     end
 
     describe "for time type" do
@@ -40,6 +44,10 @@ describe ValidatesTimeliness::Conversion do
       it "should return dummy date with time part for datetime value" do
         type_cast_value(DateTime.civil_from_format(:utc, 2010, 1, 1, 12, 34, 56), :time).should == Time.utc(2000, 1, 1, 12, 34, 56)
       end
+
+      it 'should return nil for invalid value types' do
+        type_cast_value(12, :time).should == nil
+      end
     end
 
     describe "for datetime type" do
@@ -63,6 +71,10 @@ describe ValidatesTimeliness::Conversion do
         result.should == Time.zone.local(2010, 1, 1, 23, 34, 56)
         result.zone.should == 'EST'
       end
+
+      it 'should return nil for invalid value types' do
+        type_cast_value(12, :datetime).should == nil
+      end
     end
 
     describe "ignore_usec option" do