diff --git a/sigmf/sigmffile.py b/sigmf/sigmffile.py
index d2acee9..f5f4845 100644
--- a/sigmf/sigmffile.py
+++ b/sigmf/sigmffile.py
@@ -279,11 +279,11 @@ def set_metadata(self, metadata):
             raise SigMFError("Unable to interpret provided metadata.")
 
         # if num_channels missing, default to 1
-        if self.NUM_CHANNELS_KEY not in self._metadata[self.GLOBAL_KEY]:
-            self._metadata[self.GLOBAL_KEY][self.NUM_CHANNELS_KEY] = 1
+        if self.get_global_field(self.NUM_CHANNELS_KEY) is None:
+            self.set_global_field(self.NUM_CHANNELS_KEY) = 1
 
         # set specification version to current implemented version
-        self._metadata[self.GLOBAL_KEY][self.VERSION_KEY] = __specification__
+        self.set_global_field(self.VERSION_KEY) = __specification__
 
     def set_global_info(self, new_global):
         """