--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -281,17 +281,17 @@ class CargoProvider(MachCommandBase):
         crates_and_roots = {
             'gkrust': 'toolkit/library/rust',
             'gkrust-gtest': 'toolkit/library/gtest/rust',
             'js': 'js/rust',
             'mozjs_sys': 'js/src',
             'geckodriver': 'testing/geckodriver',
         }
 
-        if crates == None:
+        if crates == None or crates == []:
             crates = ['gkrust']
 
         for crate in crates:
             root = crates_and_roots.get(crate, None)
             if not root:
                 print('Cannot locate crate %s.  Please check your spelling or '
                       'add the crate information to the list.' % crate)
                 return 1