diff --git a/caput/config.py b/caput/config.py
index e1d5b280..7075a09f 100644
--- a/caput/config.py
+++ b/caput/config.py
@@ -462,10 +462,6 @@ def _prop(config):
         loglevels = ["DEBUG", "INFO", "WARNING", "ERROR", "NOTSET"]
         for key, level in config.items():
 
-            # ignore hint at yaml file line number
-            if key == "__line__":
-                continue
-
             level = level.upper()
             if level not in loglevels:
                 raise ValueError(
@@ -488,6 +484,12 @@ def _prop(config):
     return prop
 
 
+class _line_dict(dict):
+    """A private dict subclass that also stores line numbers for debugging."""
+
+    __line__ = None
+
+
 class SafeLineLoader(SafeLoader):
     """
     YAML loader that tracks line numbers.
@@ -498,8 +500,10 @@ class SafeLineLoader(SafeLoader):
 
     def construct_mapping(self, node, deep=False):
         mapping = super(SafeLineLoader, self).construct_mapping(node, deep=deep)
+        mapping = _line_dict(mapping)
+
         # Add 1 so numbering starts at 1
-        mapping["__line__"] = node.start_mark.line + 1
+        mapping.__line__ = node.start_mark.line + 1
         return mapping
 
 
@@ -521,8 +525,8 @@ class CaputConfigError(Exception):
     def __init__(self, message, file_=None, location=None):
         self.message = message
         self.file = file_
-        if isinstance(location, dict):
-            self.line = location.get("__line__", None)
+        if isinstance(location, _line_dict):
+            self.line = location.__line__
         else:
             self.line = None
 
diff --git a/caput/pipeline.py b/caput/pipeline.py
index 74237c2d..716aabef 100644
--- a/caput/pipeline.py
+++ b/caput/pipeline.py
@@ -672,7 +672,7 @@ def _setup_task(self, task_spec):
 
         # Check that only the expected keys are in the task spec.
         for key in task_spec.keys():
-            if key not in ["type", "params", "requires", "in", "out", "__line__"]:
+            if key not in ["type", "params", "requires", "in", "out"]:
                 raise config.CaputConfigError(
                     "Task got an unexpected key '{}' in 'tasks' list.".format(key)
                 )
@@ -882,9 +882,7 @@ def cacheable(self):
     def _from_config(cls, config):
         self = cls.__new__(cls)
         # Check for unused keys, but ignore the ones not put there by the user.
-        self.read_config(
-            config, compare_keys=["versions", "pipeline_config", "__line__"]
-        )
+        self.read_config(config, compare_keys=["versions", "pipeline_config"])
         self.__init__()
         return self
 
diff --git a/caput/tests/test_metadata.py b/caput/tests/test_metadata.py
index 6c360931..9bbde7f2 100644
--- a/caput/tests/test_metadata.py
+++ b/caput/tests/test_metadata.py
@@ -21,8 +21,6 @@ def test_default_params(self):
         self.assertDictEqual(man.all_tasks_params["versions"], {})
         # remove line numbers
         pipeline_config = man.all_tasks_params["pipeline_config"]
-        del pipeline_config["__line__"]
-        del pipeline_config["pipeline"]["__line__"]
         self.assertDictEqual(
             pipeline_config,
             yaml.load(testconfig, Loader=yaml.SafeLoader),
@@ -50,8 +48,6 @@ def test_metadata_params(self):
 
         # remove line numbers
         pipeline_config = man.all_tasks_params["pipeline_config"]
-        del pipeline_config["__line__"]
-        del pipeline_config["pipeline"]["__line__"]
         self.assertDictEqual(
             pipeline_config,
             yaml.load(testconfig, Loader=yaml.SafeLoader),