diff --git a/core/dbt/config/selectors.py b/core/dbt/config/selectors.py
index 74c85aae344..6995c6a1554 100644
--- a/core/dbt/config/selectors.py
+++ b/core/dbt/config/selectors.py
@@ -1,5 +1,6 @@
 from pathlib import Path
 from typing import Dict, Any
+import yaml
 
 from hologram import ValidationError
 
@@ -33,7 +34,17 @@ def from_dict(cls, data: Dict[str, Any]) -> 'SelectorConfig':
         try:
             selector_file = SelectorFile.from_dict(data)
             selectors = parse_from_selectors_definition(selector_file)
-        except (ValidationError, RuntimeException) as exc:
+        except ValidationError as exc:
+            yaml_sel_cfg = yaml.dump(exc.instance)
+            raise DbtSelectorsError(
+                f"Could not parse selector file data: \n{yaml_sel_cfg}\n"
+                f"Valid selector description definitions: "
+                f"union, intersection, string, dictionary. No lists. "
+                f"\nhttps://docs.getdbt.com/reference/node-selection/"
+                f"yaml-selectors",
+                result_type='invalid_selector'
+            ) from exc
+        except RuntimeException as exc:
             raise DbtSelectorsError(
                 f'Could not read selector file data: {exc}',
                 result_type='invalid_selector',
diff --git a/core/dbt/graph/cli.py b/core/dbt/graph/cli.py
index b743022472c..bf8954200a0 100644
--- a/core/dbt/graph/cli.py
+++ b/core/dbt/graph/cli.py
@@ -1,5 +1,6 @@
 # special support for CLI argument parsing.
 import itertools
+import yaml
 
 from typing import (
     Dict, List, Optional, Tuple, Any, Union
@@ -165,9 +166,10 @@ def _parse_include_exclude_subdefs(
         if isinstance(definition, dict) and 'exclude' in definition:
             # do not allow multiple exclude: defs at the same level
             if diff_arg is not None:
+                yaml_sel_cfg = yaml.dump(definitions)
                 raise ValidationException(
-                    f'Got multiple exclusion definitions in definition list '
-                    f'{definitions}'
+                    f"You cannot provide multiple exclude arguments to the "
+                    f"same selector set operator:\n{yaml_sel_cfg}"
                 )
             diff_arg = _parse_exclusions(definition)
         else:
@@ -251,10 +253,15 @@ def parse_from_definition(definition: RawDefinition) -> SelectionSpec:
         return parse_intersection_definition(definition)
     elif isinstance(definition, dict):
         return parse_dict_definition(definition)
+    elif isinstance(definition, list):
+        raise ValidationException(
+            f'Selector definition root level list not allowed. Use Union to '
+            f'contain list. {definition}'
+        )
     else:
         raise ValidationException(
-            f'Expected to find str or dict, instead found '
-            f'{type(definition)}: {definition}'
+            f'Expected to find union, intersection, str or dict, instead '
+            f'found {type(definition)}: {definition}'
         )
 
 
diff --git a/test/unit/test_selector_errors.py b/test/unit/test_selector_errors.py
new file mode 100644
index 00000000000..95c4dbe0344
--- /dev/null
+++ b/test/unit/test_selector_errors.py
@@ -0,0 +1,71 @@
+from dbt.graph import cli
+import dbt.exceptions
+import textwrap
+import yaml
+import unittest
+from pprint import pprint
+from dbt.config.selectors import (
+    selector_config_from_data
+)
+
+from dbt.contracts.selection import SelectorFile
+
+
+def get_selector_dict(txt: str) -> dict:
+    txt = textwrap.dedent(txt)
+    dct = yaml.safe_load(txt)
+    return dct
+
+
+
+class SelectorUnitTest(unittest.TestCase):
+
+    def test_parse_multiple_excludes(self):
+        dct = get_selector_dict('''\
+            selectors:
+                - name: mult_excl
+                  definition:
+                    union:
+                      - method: tag
+                        value: nightly
+                      - exclude:
+                         - method: tag
+                           value: hourly
+                      - exclude:
+                         - method: tag
+                           value: daily
+            ''')
+        with self.assertRaises(dbt.exceptions.DbtSelectorsError):
+            selectors = selector_config_from_data(dct)
+
+    def test_parse_set_op_plus(self):
+        dct = get_selector_dict('''\
+            selectors:
+                - name: union_plus
+                  definition:
+                    - union:
+                       - method: tag
+                         value: nightly
+                       - exclude:
+                          - method: tag
+                            value: hourly
+                    - method: tag
+                      value: foo
+            ''')
+        with self.assertRaises(dbt.exceptions.DbtSelectorsError):
+            selectors = selector_config_from_data(dct)
+
+    def test_parse_multiple_methods(self):
+        dct = get_selector_dict('''\
+            selectors:
+                - name: mult_methods
+                  definition:
+                    - tag:hourly
+                    - tag:nightly
+                    - fqn:start
+            ''')
+        with self.assertRaises(dbt.exceptions.DbtSelectorsError):
+            selectors = selector_config_from_data(dct)
+
+
+