diff --git a/lib/interface.js b/lib/interface.js
index ad90bc0..0119dbc 100644
--- a/lib/interface.js
+++ b/lib/interface.js
@@ -47,7 +47,7 @@ function Interface() {}
 function extend()
 {
     var args      = Array.prototype.slice.call( arguments ),
-        props     = args.pop() || {},
+        props     = prop_check( args.pop() ) || {},
         base      = args.pop() || Interface,
         prototype = new base();
 
@@ -62,3 +62,26 @@ function extend()
     return new_interface;
 }
 
+
+/**
+ * Checks to ensure that only methods are being declared
+ *
+ * @param  {Object}  props  interface definition
+ *
+ * @return  {Object}  the same properties that were passed to the function
+ */
+function prop_check( props )
+{
+    for ( prop in props )
+    {
+        if ( !( props[ prop ] instanceof Function ) )
+        {
+            throw new Error(
+                "Only methods are permitted within Interface definitons"
+            );
+        }
+    }
+
+    return props;
+}
+
diff --git a/test/test-interface.js b/test/test-interface.js
index a5d959f..274f6a9 100644
--- a/test/test-interface.js
+++ b/test/test-interface.js
@@ -28,10 +28,8 @@ var assert         = require( 'assert' ),
     Class          = require( 'class' ),
     Interface      = require( 'interface' );
 
-var FooType = Interface.extend(
-{
-    foo: function( foo, bar ) {},
-});
+var FooType = Interface.extend();
+
 
 assert.ok(
     ( FooType instanceof Object ),
@@ -44,3 +42,21 @@ assert.equal(
     "Generated interface object should be frozen"
 );
 
+
+assert.throws( function()
+{
+    Interface.extend(
+    {
+        // properties (non-methods) are not permitted
+        prop: 'not permitted',
+    });
+}, Error, "Only methods are permitted within Interface definitions" );
+
+assert.doesNotThrow( function()
+{
+    Interface.extend(
+    {
+        method: function() {},
+    });
+}, Error, "Method declarations are allowed within Interface definitions" );
+