diff --git a/src/function/ArgumentValidator.java b/src/function/ArgumentValidator.java
index aef0ebb..115b3c2 100644
--- a/src/function/ArgumentValidator.java
+++ b/src/function/ArgumentValidator.java
@@ -13,6 +13,7 @@ public class ArgumentValidator {
     private String functionName;
     private Integer maximumNumberOfArguments;
     private Integer minimumNumberOfArguments;
+    private boolean isNilAcceptable;
 
     public ArgumentValidator(String functionName) {
         this.firstArgumentType = SExpression.class;
@@ -20,6 +21,7 @@ public class ArgumentValidator {
         this.functionName = functionName;
         this.minimumNumberOfArguments = null;
         this.maximumNumberOfArguments = null;
+        this.isNilAcceptable = true;
     }
 
     public void setFirstArgumentExpectedType(Class<? extends SExpression> argumentType) {
@@ -48,6 +50,10 @@ public class ArgumentValidator {
         this.maximumNumberOfArguments = exactNumberOfArguments;
     }
 
+    public void doNotAcceptNil() {
+        this.isNilAcceptable = false;
+    }
+
     public void validate(Cons argumentList) {
         validateListNotDotted(argumentList);
         validateListLength(argumentList);
@@ -80,29 +86,35 @@ public class ArgumentValidator {
     }
 
     private void validateArgumentTypes(Cons argumentList) {
-        if (!isExpectedFirstArgumentType(argumentList.getCar()))
-            throw new BadArgumentTypeException(functionName, argumentList.getCar(), firstArgumentType);
+        validateFirstArgument(argumentList);
+        validateTrailingArguments(argumentList);
+    }
 
-        validateRemainingArguments(argumentList);
+    private void validateFirstArgument(Cons argumentList) {
+        if (!isFirstArgumentValid(argumentList))
+            throw new BadArgumentTypeException(functionName, argumentList.getCar(), firstArgumentType);
+    }
+
+    private boolean isFirstArgumentValid(Cons argumentList) {
+        return argumentList.nullp() || isExpectedFirstArgumentType(argumentList.getCar());
     }
 
     private boolean isExpectedFirstArgumentType(SExpression firstArgument) {
-        return firstArgumentType.isInstance(firstArgument);
+        return firstArgumentType.isInstance(firstArgument) && !isDisallowedNil(firstArgument);
     }
 
-    private void validateRemainingArguments(Cons cons) {
-        for (cons = (Cons) cons.getCdr(); !cons.nullp(); cons = (Cons) cons.getCdr())
-            if (!isExpectedRemainingArgumentType(cons.getCar()))
+    private boolean isDisallowedNil(SExpression argument) {
+        return !isNilAcceptable && argument.nullp();
+    }
+
+    private void validateTrailingArguments(Cons argumentList) {
+        for (Cons cons = (Cons) argumentList.getCdr(); !cons.nullp(); cons = (Cons) cons.getCdr())
+            if (!isExpectedTrailingArgumentType(cons.getCar()))
                 throw new BadArgumentTypeException(functionName, cons.getCar(), trailingArgumentType);
     }
 
-    private boolean isExpectedRemainingArgumentType(SExpression remainingArgument) {
-        return trailingArgumentType.isInstance(remainingArgument);
-    }
-
-    public void validateListIsNotNil(Cons list) {
-        if (list.nullp())
-            throw new BadArgumentTypeException(functionName, Nil.getUniqueInstance(), Cons.class);
+    private boolean isExpectedTrailingArgumentType(SExpression trailingArgument) {
+        return trailingArgumentType.isInstance(trailingArgument) && !isDisallowedNil(trailingArgument);
     }
 
     public static class TooFewArgumentsException extends LispException {
diff --git a/src/function/builtin/COND.java b/src/function/builtin/COND.java
index b7a4c00..3171081 100644
--- a/src/function/builtin/COND.java
+++ b/src/function/builtin/COND.java
@@ -10,6 +10,7 @@ public class COND extends LispFunction {
     public COND() {
         this.argumentValidator = new ArgumentValidator("COND");
         this.argumentValidator.setEveryArgumentExpectedType(Cons.class);
+        this.argumentValidator.doNotAcceptNil();
     }
 
     public SExpression call(Cons argumentList) {
@@ -22,7 +23,7 @@ public class COND extends LispFunction {
         if (argumentList.nullp())
             return Nil.getUniqueInstance();
 
-        Cons clause = getFirstClauseAndValidateNotNil(argumentList);
+        Cons clause = (Cons) argumentList.getCar();
         Cons remainingClauses = (Cons) argumentList.getCdr();
         SExpression test = EVAL.eval(clause.getCar());
 
@@ -32,13 +33,6 @@ public class COND extends LispFunction {
         return callTailRecursive(remainingClauses);
     }
 
-    private Cons getFirstClauseAndValidateNotNil(Cons argumentList) {
-        Cons firstClause = (Cons) argumentList.getCar();
-        argumentValidator.validateListIsNotNil(firstClause);
-
-        return firstClause;
-    }
-
     private boolean isTestSuccessful(SExpression test) {
         return test != Nil.getUniqueInstance();
     }
diff --git a/test/function/ArgumentValidatorTester.java b/test/function/ArgumentValidatorTester.java
index 7d01618..c102956 100644
--- a/test/function/ArgumentValidatorTester.java
+++ b/test/function/ArgumentValidatorTester.java
@@ -200,13 +200,21 @@ public class ArgumentValidatorTester {
     }
 
     @Test(expected = BadArgumentTypeException.class)
-    public void validateListNotNil_ThrowsExceptionWithNilValue() {
-        validator.validateListIsNotNil(Nil.getUniqueInstance());
+    public void doNotAcceptNil_ThrowsExceptionOnNilArgument() {
+        validator.doNotAcceptNil();
+        validator.validate(new Cons(Symbol.T, new Cons(Nil.getUniqueInstance(), Nil.getUniqueInstance())));
     }
 
     @Test
-    public void validateListNotNil_DoesNotThrowExceptionWithNonNilValue() {
-        validator.validateListIsNotNil(new Cons(Nil.getUniqueInstance(), Nil.getUniqueInstance()));
+    public void doNotAcceptNil_AllowsEmptyArgumentList() {
+        validator.doNotAcceptNil();
+        validator.validate(Nil.getUniqueInstance());
+    }
+
+    @Test
+    public void doNotAcceptNil_AllowsProperList() {
+        validator.doNotAcceptNil();
+        validator.validate(new Cons(Symbol.T, new Cons(Symbol.T, Nil.getUniqueInstance())));
     }
 
 }
diff --git a/test/function/builtin/APPLYTester.java b/test/function/builtin/APPLYTester.java
index 9b1b62d..70338c3 100644
--- a/test/function/builtin/APPLYTester.java
+++ b/test/function/builtin/APPLYTester.java
@@ -58,4 +58,9 @@ public class APPLYTester {
         evaluateString("(apply '1)");
     }
 
+    @Test(expected = DottedArgumentListException.class)
+    public void testCondWithDottedArgumentList_ThrowsException() {
+        evaluateString("(apply 'apply (cons 'T 'T))");
+    }
+
 }
diff --git a/test/function/builtin/CONDTester.java b/test/function/builtin/CONDTester.java
index d3034a3..a6a1697 100644
--- a/test/function/builtin/CONDTester.java
+++ b/test/function/builtin/CONDTester.java
@@ -4,7 +4,7 @@ import static testutil.TestUtilities.*;
 
 import org.junit.Test;
 
-import function.ArgumentValidator.BadArgumentTypeException;
+import function.ArgumentValidator.*;
 
 public class CONDTester {
 
@@ -74,4 +74,9 @@ public class CONDTester {
         evaluateString("(cond o)");
     }
 
+    @Test(expected = DottedArgumentListException.class)
+    public void testCondWithDottedArgumentList_ThrowsException() {
+        evaluateString("(apply 'cond (cons '(nil T) 'b))");
+    }
+
 }