diff --git a/haystack/preview/utils/filters.py b/haystack/preview/utils/filters.py
index e5b464c86b..61016f993f 100644
--- a/haystack/preview/utils/filters.py
+++ b/haystack/preview/utils/filters.py
@@ -330,6 +330,10 @@ def convert(filters: Dict[str, Any]) -> Dict[str, Any]:
     }
     ```
     """
+    if not isinstance(filters, dict):
+        msg = f"Can't convert filters from type '{type(filters)}'"
+        raise ValueError(msg)
+
     converted = _internal_convert(filters)
     if "conditions" not in converted:
         # This is done to handle a corner case when filter is really simple like so:
@@ -354,23 +358,35 @@ def _internal_convert(filters: Union[List[Any], Dict[str, Any]], previous_key=No
         return _handle_non_dict(filters, previous_key)
 
     for key, value in filters.items():
-        if key not in ALL_OPERATORS:
+        if (
+            previous_key is not None
+            and previous_key not in ALL_LEGACY_OPERATORS_MAPPING
+            and key not in ALL_LEGACY_OPERATORS_MAPPING
+        ):
+            msg = f"This filter ({filters}) seems to be malformed."
+            raise FilterError(msg)
+        if key not in ALL_LEGACY_OPERATORS_MAPPING:
             converted = _internal_convert(value, previous_key=key)
             if isinstance(converted, list):
                 conditions.extend(converted)
             else:
                 conditions.append(converted)
-        elif key in LOGIC_OPERATORS:
-            if previous_key not in ALL_OPERATORS and isinstance(value, list):
+        elif key in LEGACY_LOGICAL_OPERATORS_MAPPING:
+            if previous_key not in ALL_LEGACY_OPERATORS_MAPPING and isinstance(value, list):
                 converted = [_internal_convert({previous_key: v}) for v in value]
-                conditions.append({"operator": ALL_OPERATORS[key], "conditions": converted})
+                conditions.append({"operator": ALL_LEGACY_OPERATORS_MAPPING[key], "conditions": converted})
             else:
                 converted = _internal_convert(value, previous_key=key)
+                if key == "$not" and type(converted) not in [dict, list]:
+                    # This handles a corner when '$not' is used like this:
+                    # '{"page": {"$not": 102}}'
+                    # Without this check we would miss the implicit '$eq'
+                    converted = {"field": previous_key, "operator": "==", "value": value}
                 if not isinstance(converted, list):
                     converted = [converted]
-                conditions.append({"operator": ALL_OPERATORS[key], "conditions": converted})
-        elif key in COMPARISON_OPERATORS:
-            conditions.append({"field": previous_key, "operator": ALL_OPERATORS[key], "value": value})
+                conditions.append({"operator": ALL_LEGACY_OPERATORS_MAPPING[key], "conditions": converted})
+        elif key in LEGACY_COMPARISON_OPERATORS_MAPPING:
+            conditions.append({"field": previous_key, "operator": ALL_LEGACY_OPERATORS_MAPPING[key], "value": value})
 
     if len(conditions) == 1:
         return conditions[0]
@@ -382,23 +398,23 @@ def _internal_convert(filters: Union[List[Any], Dict[str, Any]], previous_key=No
 
 
 def _handle_list(filters, previous_key):
-    if previous_key in LOGIC_OPERATORS:
+    if previous_key in LEGACY_LOGICAL_OPERATORS_MAPPING:
         return [_internal_convert(f) for f in filters]
-    elif previous_key not in COMPARISON_OPERATORS:
+    elif previous_key not in LEGACY_COMPARISON_OPERATORS_MAPPING:
         return {"field": previous_key, "operator": "in", "value": filters}
     return None
 
 
 def _handle_non_dict(filters, previous_key):
-    if previous_key not in ALL_OPERATORS:
+    if previous_key not in ALL_LEGACY_OPERATORS_MAPPING:
         return {"field": previous_key, "operator": "==", "value": filters}
     return filters
 
 
 # Operator mappings from legacy style to new one
-LOGIC_OPERATORS = {"$and": "AND", "$or": "OR", "$not": "NOT"}
+LEGACY_LOGICAL_OPERATORS_MAPPING = {"$and": "AND", "$or": "OR", "$not": "NOT"}
 
-COMPARISON_OPERATORS = {
+LEGACY_COMPARISON_OPERATORS_MAPPING = {
     "$eq": "==",
     "$ne": "!=",
     "$gt": ">",
@@ -409,4 +425,4 @@ def _handle_non_dict(filters, previous_key):
     "$nin": "not in",
 }
 
-ALL_OPERATORS = {**LOGIC_OPERATORS, **COMPARISON_OPERATORS}
+ALL_LEGACY_OPERATORS_MAPPING = {**LEGACY_LOGICAL_OPERATORS_MAPPING, **LEGACY_COMPARISON_OPERATORS_MAPPING}
diff --git a/test/preview/utils/test_filters.py b/test/preview/utils/test_filters.py
index 076c83e571..2ca7826a86 100644
--- a/test/preview/utils/test_filters.py
+++ b/test/preview/utils/test_filters.py
@@ -653,9 +653,27 @@ def test_not(self):
         },
         id="Root explicit $not",
     ),
+    pytest.param(
+        {"page": {"$not": 102}},
+        {"operator": "NOT", "conditions": [{"field": "page", "operator": "==", "value": 102}]},
+        id="Explicit $not with implicit $eq",
+    ),
 ]
 
 
 @pytest.mark.parametrize("old_style, new_style", filters_data)
 def test_convert(old_style, new_style):
     assert convert(old_style) == new_style
+
+
+def test_convert_with_incorrect_input_type():
+    with pytest.raises(ValueError):
+        convert("some string")
+
+
+def test_convert_with_incorrect_filter_nesting():
+    with pytest.raises(FilterError):
+        convert({"number": {"page": "100"}})
+
+    with pytest.raises(FilterError):
+        convert({"number": {"page": {"chapter": "intro"}}})