Throw error if item content is invalid

lib/filesystem.js

@@ -138,12 +138,14 @@ function populate(directory, name, obj) {
   } else if (typeof obj === 'function') {
     // item factory
     item = obj();
-  } else {
+  } else if (typeof obj === 'object') {
     // directory with more to populate
     item = new Directory();
     for (var key in obj) {
       populate(item, key, obj[key]);
     }
+  } else {
+    throw new Error('Unsupported type: ' + typeof obj + ' of item ' + name);
   }
   /**
    * Special exception for redundant adding of empty directories.

test/lib/filesystem.spec.js

@@ -311,4 +311,26 @@ describe('FileSystem.create', function() {
      */
     assert.equal(system.getItem('/dir-a.0/dir-b.0/symlink-c.0').links, 1);
   });
+
+  it('throws if item content is not valid type', function() {
+    assert.throws(function() {
+      FileSystem.create({
+        '/dir-a.0': {
+          'dir-b.0': {
+            'file-c.0': undefined
+          }
+        }
+      });
+    });
+
+    assert.throws(function() {
+      FileSystem.create({
+        '/dir-a.0': {
+          'dir-b.0': {
+            'file-c.0': 123
+          }
+        }
+      });
+    });
+  });
 });