diff --git a/lib/contracts/builtin_contracts.rb b/lib/contracts/builtin_contracts.rb
index 0164b22..9c60a18 100644
--- a/lib/contracts/builtin_contracts.rb
+++ b/lib/contracts/builtin_contracts.rb
@@ -29,14 +29,14 @@ def self.valid? val
   # Check that an argument is a positive number.
   class Pos
     def self.valid? val
-      val > 0
+      val && val > 0
     end
   end
 
   # Check that an argument is a negative number.
   class Neg
     def self.valid? val
-      val < 0
+      val && val < 0
     end
   end
 
diff --git a/spec/builtin_contracts_spec.rb b/spec/builtin_contracts_spec.rb
index 9987506..01d9379 100644
--- a/spec/builtin_contracts_spec.rb
+++ b/spec/builtin_contracts_spec.rb
@@ -29,6 +29,10 @@
     it "should fail for negative numbers" do
       expect { @o.pos_test(-1) }.to raise_error(ContractError)
     end
+
+    it "should fail for nil" do
+      expect { @o.pos_test(nil) }.to raise_error(ContractError)
+    end
   end
 
   describe "Neg:" do
@@ -43,6 +47,10 @@
     it "should fail for positive numbers" do
       expect { @o.neg_test(1) }.to raise_error(ContractError)
     end
+
+    it "should fail for nil" do
+      expect { @o.neg_test(nil) }.to raise_error(ContractError)
+    end
   end
 
   describe "Nat:" do
@@ -61,6 +69,10 @@
     it "should fail for negative numbers" do
       expect { @o.nat_test(-1) }.to raise_error(ContractError)
     end
+
+    it "should fail for nil" do
+      expect { @o.nat_test(nil) }.to raise_error(ContractError)
+    end
   end
 
   describe "Any:" do