diff --git a/src/function/ArgumentValidator.java b/src/function/ArgumentValidator.java
index a79f1b3..96a1a78 100644
--- a/src/function/ArgumentValidator.java
+++ b/src/function/ArgumentValidator.java
@@ -12,18 +12,20 @@ public class ArgumentValidator {
 
     private Class<? extends SExpression> firstArgumentType;
     private Class<? extends SExpression> trailingArgumentType;
+    private Class<? extends SExpression> excludedFirstArgumentType;
+    private Class<? extends SExpression> excludedTrailingArgumentType;
     private String functionName;
     private BigInteger maximumNumberOfArguments;
     private BigInteger minimumNumberOfArguments;
-    private boolean isNilAcceptable;
 
     public ArgumentValidator(String functionName) {
         this.firstArgumentType = SExpression.class;
         this.trailingArgumentType = SExpression.class;
+        this.excludedFirstArgumentType = null;
+        this.excludedTrailingArgumentType = null;
         this.functionName = functionName;
         this.minimumNumberOfArguments = null;
         this.maximumNumberOfArguments = null;
-        this.isNilAcceptable = true;
     }
 
     public void setFirstArgumentExpectedType(Class<? extends SExpression> argumentType) {
@@ -38,6 +40,19 @@ public class ArgumentValidator {
         this.firstArgumentType = argumentType;
         this.trailingArgumentType = argumentType;
     }
+    
+    public void setFirstArgumentExcludedType(Class<? extends SExpression> argumentType) {
+        this.excludedFirstArgumentType = argumentType;
+    }
+
+    public void setTrailingArgumentExcludedType(Class<? extends SExpression> argumentType) {
+        this.excludedTrailingArgumentType = argumentType;
+    }
+
+    public void setEveryArgumentExcludedType(Class<? extends SExpression> argumentType) {
+        this.excludedFirstArgumentType = argumentType;
+        this.excludedTrailingArgumentType = argumentType;
+    } 
 
     public void setMaximumNumberOfArguments(int maximumNumberOfArguments) {
         this.maximumNumberOfArguments = BigInteger.valueOf(maximumNumberOfArguments);
@@ -52,10 +67,6 @@ public class ArgumentValidator {
         this.maximumNumberOfArguments = BigInteger.valueOf(exactNumberOfArguments);
     }
 
-    public void doNotAcceptNil() {
-        this.isNilAcceptable = false;
-    }
-
     public void validate(Cons argumentList) {
         validateListNotDotted(argumentList);
         validateListLength(argumentList);
@@ -118,11 +129,11 @@ public class ArgumentValidator {
     }
 
     private boolean isExpectedFirstArgumentType(SExpression firstArgument) {
-        return firstArgumentType.isInstance(firstArgument) && !isDisallowedNil(firstArgument);
+        return firstArgumentType.isInstance(firstArgument) && !isExcludedFirstArgumentType(firstArgument);
     }
 
-    private boolean isDisallowedNil(SExpression argument) {
-        return !isNilAcceptable && argument.isNull();
+    private boolean isExcludedFirstArgumentType(SExpression firstArgument) {
+        return excludedFirstArgumentType != null && excludedFirstArgumentType.isInstance(firstArgument);
     }
 
     private void validateTrailingArguments(Cons argumentList) {
@@ -132,7 +143,11 @@ public class ArgumentValidator {
     }
 
     private boolean isExpectedTrailingArgumentType(SExpression trailingArgument) {
-        return trailingArgumentType.isInstance(trailingArgument) && !isDisallowedNil(trailingArgument);
+        return trailingArgumentType.isInstance(trailingArgument) && !isExcludedTrailingArgumentType(trailingArgument);
+    }
+
+    private boolean isExcludedTrailingArgumentType(SExpression trailingArgument) {
+        return excludedTrailingArgumentType != null && excludedTrailingArgumentType.isInstance(trailingArgument);
     }
 
     public static class TooFewArgumentsException extends LispException {
diff --git a/src/function/builtin/special/CASE.java b/src/function/builtin/special/CASE.java
index fc85b97..8750131 100644
--- a/src/function/builtin/special/CASE.java
+++ b/src/function/builtin/special/CASE.java
@@ -17,7 +17,7 @@ public class CASE extends LispSpecialFunction {
         this.argumentValidator = new ArgumentValidator("CASE");
         this.argumentValidator.setMinimumNumberOfArguments(1);
         this.argumentValidator.setTrailingArgumentExpectedType(Cons.class);
-        this.argumentValidator.doNotAcceptNil();
+        this.argumentValidator.setTrailingArgumentExcludedType(Nil.class);
     }
 
     public SExpression call(Cons argumentList) {
diff --git a/src/function/builtin/special/COND.java b/src/function/builtin/special/COND.java
index b34f1c7..5ec47c7 100644
--- a/src/function/builtin/special/COND.java
+++ b/src/function/builtin/special/COND.java
@@ -14,7 +14,7 @@ public class COND extends LispSpecialFunction {
     public COND() {
         this.argumentValidator = new ArgumentValidator("COND");
         this.argumentValidator.setEveryArgumentExpectedType(Cons.class);
-        this.argumentValidator.doNotAcceptNil();
+        this.argumentValidator.setEveryArgumentExcludedType(Nil.class);
     }
 
     public SExpression call(Cons argumentList) {
diff --git a/test/function/ArgumentValidatorTester.java b/test/function/ArgumentValidatorTester.java
index 5ea4969..2b7a8e6 100644
--- a/test/function/ArgumentValidatorTester.java
+++ b/test/function/ArgumentValidatorTester.java
@@ -74,46 +74,28 @@ public class ArgumentValidatorTester {
     }
 
     @Test
-    public void tooManyArgumentsException_HasCorrectSeverity() {
+    public void tooManyArgumentsException_HasCorrectAttributes() {
         TooManyArgumentsException e = new TooManyArgumentsException("TEST", NIL);
 
         assertEquals(ERROR, e.getSeverity());
-    }
-
-    @Test
-    public void tooManyArgumentsException_HasMessageText() {
-        TooManyArgumentsException e = new TooManyArgumentsException("TEST", NIL);
-
         assertNotNull(e.getMessage());
         assertTrue(e.getMessage().length() > 0);
     }
 
     @Test
-    public void tooFewArgumentsException_HasCorrectSeverity() {
+    public void tooFewArgumentsException_HasCorrectAttributes() {
         TooFewArgumentsException e = new TooFewArgumentsException("TEST", NIL);
 
         assertEquals(ERROR, e.getSeverity());
-    }
-
-    @Test
-    public void tooFewArgumentsException_HasMessageText() {
-        TooFewArgumentsException e = new TooFewArgumentsException("TEST", NIL);
-
         assertNotNull(e.getMessage());
         assertTrue(e.getMessage().length() > 0);
     }
 
     @Test
-    public void BadArgumentTypeException_HasCorrectSeverity() {
+    public void badArgumentTypeException_HasCorrectAttributes() {
         BadArgumentTypeException e = new BadArgumentTypeException("TEST", NIL, SExpression.class);
 
         assertEquals(ERROR, e.getSeverity());
-    }
-
-    @Test
-    public void BadArgumentTypeException_HasMessageText() {
-        BadArgumentTypeException e = new BadArgumentTypeException("TEST", NIL, SExpression.class);
-
         assertNotNull(e.getMessage());
         assertTrue(e.getMessage().length() > 0);
     }
@@ -201,22 +183,40 @@ public class ArgumentValidatorTester {
         assertTrue(e.getMessage().length() > 0);
     }
 
-    @Test(expected = BadArgumentTypeException.class)
-    public void doNotAcceptNil_ThrowsExceptionOnNilArgument() {
-        validator.doNotAcceptNil();
+    @Test
+    public void excludedFirstArgumentType_DoesNotAffectTrailingArguments() {
+        validator.setFirstArgumentExcludedType(Nil.class);
         validator.validate(new Cons(T, new Cons(NIL, NIL)));
     }
 
     @Test
-    public void doNotAcceptNil_AllowsEmptyArgumentList() {
-        validator.doNotAcceptNil();
-        validator.validate(NIL);
+    public void excludedTrailingArgumentType_DoesNotAffectFirstArgument() {
+        validator.setTrailingArgumentExcludedType(Nil.class);
+        validator.validate(new Cons(NIL, new Cons(T, NIL)));
     }
 
-    @Test
-    public void doNotAcceptNil_AllowsProperList() {
-        validator.doNotAcceptNil();
-        validator.validate(new Cons(T, new Cons(T, NIL)));
+    @Test(expected = BadArgumentTypeException.class)
+    public void excludedFirstArgumentType_ThrowsException() {
+        validator.setFirstArgumentExcludedType(Nil.class);
+        validator.validate(new Cons(NIL, new Cons(T, NIL)));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void excludedTrailingArgumentType_ThrowsException() {
+        validator.setTrailingArgumentExcludedType(Nil.class);
+        validator.validate(new Cons(T, new Cons(NIL, NIL)));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void excludedArgumentType_ThrowsExceptionOnFirstArgument() {
+        validator.setEveryArgumentExcludedType(Nil.class);
+        validator.validate(new Cons(NIL, new Cons(T, NIL)));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void excludedArgumentType_ThrowsExceptionOnTrailingArgument() {
+        validator.setEveryArgumentExcludedType(Nil.class);
+        validator.validate(new Cons(T, new Cons(NIL, NIL)));
     }
 
 }