diff --git a/lib/model.js b/lib/model.js
index 7d5f2c8..70f7d6e 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -125,8 +125,20 @@ function deepCopy (obj) {
 }
 
 
+/**
+ * Set field to value in a model
+ * Create it if it doesn't exist
+ * @param {Object} obj The model to set a field for
+ * @param {Object} mod mod is of the form { k: v }. k is the field, v its value
+ *                     k can contain dots, in that case that means we will set a subfield
+ */
+modifierFunctions.$set = function (obj, mod) {
+  var field, value;
+
+
+};
+
 modifierFunctions.$inc = function () {};
-modifierFunctions.$set = function () {};
 
 
 /**
@@ -154,7 +166,19 @@ function modify (obj, updateQuery) {
     // Apply modifiers
     modifiers = _.uniq(keys);
     modifiers.forEach(function (m) {
+      var keys;
+
       if (!modifierFunctions[m]) { throw "Unknown modifier " + m; }
+
+      try {
+        keys = Object.keys(updateQuery[m]);
+      } catch (e) {
+        throw "Modifier " + m + "'s argument must be an object";
+      }
+
+      keys.forEach(function (k) {
+        modifierFunctions[m](obj, k, updateQuery[m][k]);
+      });
     });
   }
 
diff --git a/test/model.test.js b/test/model.test.js
index 1984bec..e2fc1dd 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -240,6 +240,15 @@ describe('Model', function () {
       }).should.throw();
     });
 
+    it('Throw an error if a modifier is used with a non-object argument', function () {
+      var obj = { some: 'thing' }
+        , updateQuery = { $set: 'this exists' };
+
+      (function () {
+        model.modify(obj, updateQuery);
+      }).should.throw();
+    });
+
   });   // ==== End of 'Modifying documents' ==== //
 
 });