diff --git a/lisp/lang/functions.lisp b/lisp/lang/functions.lisp
index c684278..985b765 100644
--- a/lisp/lang/functions.lisp
+++ b/lisp/lang/functions.lisp
@@ -37,13 +37,6 @@
         accumulator))
     accumulator))
 
-; (defun append (list-one list-two)
-;   (append-tail (reverse list-one) list-two))
-
-; (defun append-tail (reversed-list list-two)
-;   (if (null reversed-list) list-two
-;     (recur (rest reversed-list) (cons (car reversed-list) list-two))))
-
 (defun reverse (the-list)
   (reverse-tail () the-list))
 
diff --git a/lisp/list/append.lisp b/lisp/list/append.lisp
new file mode 100644
index 0000000..7ceba01
--- /dev/null
+++ b/lisp/list/append.lisp
@@ -0,0 +1,7 @@
+
+(defun my-append (list-one list-two)
+  (append-tail (reverse list-one) list-two))
+
+(defun append-tail (reversed-list list-two)
+  (if (null reversed-list) list-two
+    (recur (rest reversed-list) (cons (car reversed-list) list-two))))
diff --git a/src/function/builtin/cons/LENGTH.java b/src/function/builtin/cons/LENGTH.java
index 2acade0..1046467 100644
--- a/src/function/builtin/cons/LENGTH.java
+++ b/src/function/builtin/cons/LENGTH.java
@@ -28,16 +28,19 @@ public class LENGTH extends LispFunction {
     }
 
     private ArgumentValidator argumentValidator;
+    private ArgumentValidator properListValidator;
 
     public LENGTH(String name) {
         this.argumentValidator = new ArgumentValidator(name);
         this.argumentValidator.setExactNumberOfArguments(1);
         this.argumentValidator.setEveryArgumentExpectedType(Cons.class);
+        this.properListValidator = new ArgumentValidator(name + "|list|");
     }
 
     @Override
     public LispNumber call(Cons argumentList) {
         argumentValidator.validate(argumentList);
+        properListValidator.validate((Cons) argumentList.getFirst());
 
         return callTailRecursive(BigInteger.ZERO, argumentList).invoke();
     }
diff --git a/test/function/builtin/cons/LENGTHTest.java b/test/function/builtin/cons/LENGTHTest.java
index 2968159..f2acc3d 100644
--- a/test/function/builtin/cons/LENGTHTest.java
+++ b/test/function/builtin/cons/LENGTHTest.java
@@ -7,6 +7,7 @@ import static testutil.TestUtilities.parseString;
 import org.junit.Test;
 
 import function.ArgumentValidator.BadArgumentTypeException;
+import function.ArgumentValidator.DottedArgumentListException;
 import function.ArgumentValidator.TooFewArgumentsException;
 import function.ArgumentValidator.TooManyArgumentsException;
 import testutil.SymbolAndFunctionCleaner;
@@ -49,4 +50,9 @@ public class LENGTHTest extends SymbolAndFunctionCleaner {
         evaluateString("(length)");
     }
 
+    @Test(expected = DottedArgumentListException.class)
+    public void lengthWithDottedList() {
+        evaluateString("(length (cons 1 2))");
+    }
+
 }