diff --git a/lib/lisbn/lisbn.rb b/lib/lisbn/lisbn.rb
index 6d25ce0..c6446ff 100644
--- a/lib/lisbn/lisbn.rb
+++ b/lib/lisbn/lisbn.rb
@@ -6,18 +6,30 @@ def isbn
 
   # Returns true if the ISBN is valid, false otherwise.
   def valid?
+    case isbn.length
+    when 10
+      valid_isbn_10?
+    when 13
+      valid_isbn_13?
+    else
+      false
+    end
+  end
+
+  # Returns true if the ISBN is valid, false otherwise.
+  def valid_checksum?
     case isbn.length
       when 10
-        valid_isbn_10?
+        valid_checksum_isbn_10?
       when 13
-        valid_isbn_13?
+        valid_checksum_isbn_13?
       else
         false
     end
   end
 
   def isbn_with_dash
-    if valid_isbn_13? && parts
+    if valid_checksum_isbn_13? && parts
       parts.join("-")
     elsif isbn.length > 3
       isbn[0..-2] + "-" + isbn[-1]
@@ -39,7 +51,7 @@ def isbn10
   # Returns a valid ISBN in ISBN-13 format.
   # Returns nil if the ISBN is invalid.
   def isbn13
-    return unless valid?
+    return unless valid_checksum?
     return isbn if isbn.length == 13
 
     '978' + isbn[0..-2] + isbn_13_checksum
@@ -138,19 +150,27 @@ def isbn_13_checksum
     (10 - sum % 10).to_s[-1]
   end
 
-  cache_method :isbn, :valid?, :isbn10, :isbn13, :parts, :isbn_10_checksum, :isbn_13_checksum
+  cache_method :isbn, :valid?, :valid_checksum?, :isbn10, :isbn13, :parts, :isbn_10_checksum, :isbn_13_checksum
 
 private
 
-  def valid_isbn_10?
+  def valid_checksum_isbn_10?
     return false unless isbn.match(/^[0-9]{9}[0-9X]$/)
     isbn[-1..-1] == isbn_10_checksum
   end
 
-  def valid_isbn_13?
+  def valid_checksum_isbn_13?
     return false unless isbn.match(/^[0-9]{13}$/)
     isbn[-1..-1] == isbn_13_checksum
   end
 
+  def valid_isbn_10?
+    !parts(4).blank?
+  end
+
+  def valid_isbn_13?
+    !parts.blank?
+  end
+
   RANGES = YAML::load_file(File.dirname(__FILE__) + "/../../data/ranges.yml")
 end
diff --git a/spec/lisbn_spec.rb b/spec/lisbn_spec.rb
index 4beedba..7742213 100644
--- a/spec/lisbn_spec.rb
+++ b/spec/lisbn_spec.rb
@@ -44,6 +44,16 @@
       isbn.valid?.should be false
     end
 
+    it "recognizes an invalid ISBN13 with valid checksum" do
+      isbn = Lisbn.new("8830114722800")
+      isbn.valid?.should be false
+    end
+
+    it "recognizes an invalid ISBN10 with valid checksum" do
+      isbn = Lisbn.new("8830114722800")
+      isbn.valid?.should be false
+    end
+
     it "returns false for improperly-formatted ISBNs" do
       isbn = Lisbn.new("97800000X0002")
       isbn.valid?.should be false
@@ -55,6 +65,54 @@
     end
   end
 
+
+  describe "#valid_checksum?" do
+    it "recognizes a valid checksum for ISBN10 format" do
+      isbn = Lisbn.new("0123456789")
+      isbn.valid_checksum?.should be true
+    end
+
+    it "recognizes a valid checksum for ISBN10 with X" do
+      isbn = Lisbn.new("160459411X")
+      isbn.valid_checksum?.should be true
+    end
+
+    it "recognizes a valid checksum for ISBN10 with 0" do
+      isbn = Lisbn.new("0679405070")
+      isbn.valid_checksum?.should be true
+    end
+
+    it "recognizes an invalid ISBN10 checksum" do
+      isbn = Lisbn.new("0123546789")
+      isbn.valid_checksum?.should be false
+    end
+
+    it "recognizes a valid checksum for ISBN13 format" do
+      isbn = Lisbn.new("9780000000002")
+      isbn.valid_checksum?.should be true
+    end
+
+    it "recognizes a valid checksum for ISBN13 with 0" do
+      isbn = Lisbn.new("9780062870780")
+      isbn.valid_checksum?.should be true
+    end
+
+    it "recognizes an invalid checksum for ISBN13" do
+      isbn = Lisbn.new("9780000000003")
+      isbn.valid_checksum?.should be false
+    end
+
+    it "returns false for improperly-formatted ISBNs" do
+      isbn = Lisbn.new("97800000X0002")
+      isbn.valid_checksum?.should be false
+    end
+
+    it "regards anything not 10 or 13 digits as invalid" do
+      isbn = Lisbn.new("")
+      isbn.valid_checksum?.should be false
+    end
+  end
+
   describe "#isbn_with_dash" do
     subject { Lisbn.new(isbn) }
 
@@ -87,13 +145,13 @@
     subject { Lisbn.new("9780000000002") }
 
     it "returns nil if invalid" do
-      subject.stub(:valid? => false)
+      subject.stub(:valid_checksum? => false)
       subject.isbn10.should be_nil
     end
 
     it "returns nil if the ISBN is 13-digits and isn't in the 978 GS1" do
       lisbn = Lisbn.new("9790000000003")
-      lisbn.stub(:valid? => true)
+      lisbn.stub(:valid_checksum? => true)
       lisbn.isbn10.should be_nil
     end
 
@@ -103,7 +161,7 @@
 
     it "returns the isbn if it's 10 digits" do
       lisbn = Lisbn.new("0000000000")
-      lisbn.stub(:valid? => true)
+      lisbn.stub(:valid_checksum? => true)
       lisbn.should_not_receive(:isbn_10_checksum)
       lisbn.isbn10.should == "0000000000"
     end
@@ -113,7 +171,7 @@
     subject { Lisbn.new("0000000000") }
 
     it "returns nil if invalid" do
-      subject.stub(:valid? => false)
+      subject.stub(:valid_checksum? => false)
       subject.isbn13.should be_nil
     end
 
@@ -123,7 +181,7 @@
 
     it "returns the isbn if it's 13 digits" do
       lisbn = Lisbn.new("9780000000002")
-      lisbn.stub(:valid? => true)
+      lisbn.stub(:valid_checksum? => true)
       lisbn.should_not_receive(:isbn_13_checksum)
       lisbn.isbn13.should == "9780000000002"
     end