diff --git a/lib/puppet/parser/functions/is_valid_mac_address.rb b/lib/puppet/parser/functions/is_valid_mac_address.rb
index 0b91d0d..a00d874 100644
--- a/lib/puppet/parser/functions/is_valid_mac_address.rb
+++ b/lib/puppet/parser/functions/is_valid_mac_address.rb
@@ -12,6 +12,14 @@ module Puppet::Parser::Functions
         "given #{arguments.size} for 1")
     end
 
+    mac = arguments[0]
+
+    if /^[a-fA-F0-9]{1,2}:[a-fA-F0-9]{1,2}:[a-fA-F0-9]{1,2}:[a-fA-F0-9]{1,2}:[a-fA-F0-9]{1,2}:[a-fA-F0-9]{1,2}$/.match(mac) then
+      return true
+    else
+      return false
+    end
+
   end
 end
 
diff --git a/spec/unit/parser/functions/is_valid_mac_address_spec.rb b/spec/unit/parser/functions/is_valid_mac_address_spec.rb
index c4eb468..62e6fee 100644
--- a/spec/unit/parser/functions/is_valid_mac_address_spec.rb
+++ b/spec/unit/parser/functions/is_valid_mac_address_spec.rb
@@ -19,12 +19,17 @@ describe "the is_valid_mac_address function" do
   end
 
   it "should return true if a valid mac address" do
-    result = @scope.function_is_valid_mac_address("00:a0:1f:12:7f:a0")
+    result = @scope.function_is_valid_mac_address(["00:a0:1f:12:7f:a0"])
     result.should(eq(true))
   end
 
+  it "should return false if octets are out of range" do
+    result = @scope.function_is_valid_mac_address(["00:a0:1f:12:7f:g0"])
+    result.should(eq(false))
+  end
+
   it "should return false if not valid" do
-    result = @scope.function_is_valid_mac_address("not valid")
+    result = @scope.function_is_valid_mac_address(["not valid"])
     result.should(eq(false))
   end