diff --git a/README.md b/README.md
index e8b7401..89d2b22 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@ teacher_that_hasChild_male = OWLObjectIntersectionOf([hasChild_male, teacher])
 # You can render and print owl class expressions in description logics syntax (and vice-versa)
 print(owl_expression_to_dl(teacher_that_hasChild_male))
 # (∃ hasChild.male) ⊓ teacher
-print(owl_expression_to_sparql("?x", teacher_that_hasChild_male))
+print(owl_expression_to_sparql(teacher_that_hasChild_male))
 #  SELECT DISTINCT ?x WHERE {  ?x <http://example.com/society#hasChild> ?s_1 . ?s_1 a <http://example.com/society#male> . ?x a <http://example.com/society#teacher> .  } }
 ```
 
diff --git a/docs/usage/usage_examples.md b/docs/usage/usage_examples.md
index 531b249..41413dc 100644
--- a/docs/usage/usage_examples.md
+++ b/docs/usage/usage_examples.md
@@ -101,7 +101,7 @@ from owlapy import owl_expression_to_sparql, owl_expression_to_dl, owl_expressio
 print(owl_expression_to_dl(ce))
 # Result: male ⊓ (≥ 1 hasChild.person)
 
-print(owl_expression_to_sparql(expression=ce))
+print(owl_expression_to_sparql(ce))
 # Result: SELECT DISTINCT ?x WHERE { ?x a <http://example.com/family#male> . { SELECT ?x WHERE { ?x <http://example.com/family#hasChild> ?s_1 . ?s_1 a <http://example.com/family#person> .  } GROUP BY ?x HAVING ( COUNT ( ?s_1 ) >= 1 ) } }
 
 print(owl_expression_to_manchester(ce))
diff --git a/owlapy/class_expression/restriction.py b/owlapy/class_expression/restriction.py
index 6488181..04e7a4b 100644
--- a/owlapy/class_expression/restriction.py
+++ b/owlapy/class_expression/restriction.py
@@ -461,6 +461,7 @@ class OWLQuantifiedDataRestriction(OWLQuantifiedRestriction[OWLDataRange],
     _filler: OWLDataRange
 
     def __init__(self, filler: OWLDataRange):
+        assert isinstance(filler, OWLDataRange), "filler must be an OWLDataRange"
         self._filler = filler
 
     def get_filler(self) -> OWLDataRange:
@@ -478,6 +479,7 @@ class OWLDataCardinalityRestriction(OWLCardinalityRestriction[OWLDataRange],
 
     @abstractmethod
     def __init__(self, cardinality: int, property: OWLDataPropertyExpression, filler: OWLDataRange):
+        assert isinstance(filler, OWLDataRange), "filler must be an OWLDataRange"
         super().__init__(cardinality, filler)
         self._property = property
 
diff --git a/owlapy/converter.py b/owlapy/converter.py
index 3751db5..c83dbe5 100644
--- a/owlapy/converter.py
+++ b/owlapy/converter.py
@@ -498,7 +498,7 @@ def _(self, ce: OWLDataAllValuesFrom):
     def _(self, ce: OWLDataHasValue):
         property_expression = ce.get_property()
         value = ce.get_filler()
-        assert isinstance(value, OWLDataProperty)
+        assert isinstance(value, OWLLiteral)
         self.append_triple(self.current_variable, property_expression, value)
 
     @process.register
@@ -587,10 +587,10 @@ def as_query(self,
 converter = Owl2SparqlConverter()
 
 
-def owl_expression_to_sparql(root_variable: str = "?x",
-                             expression: OWLClassExpression = None,
+def owl_expression_to_sparql(expression: OWLClassExpression = None,
+                             root_variable: str = "?x",
                              values: Optional[Iterable[OWLNamedIndividual]] = None,
-                             named_individuals: bool = False)->str:
+                             named_individuals: bool = False) -> str:
     """Convert an OWL Class Expression (https://www.w3.org/TR/owl2-syntax/#Class_Expressions) into a SPARQL query
      root variable: the variable that will be projected
      expression: the class expression to be transformed to a SPARQL query
diff --git a/tests/test_examples.py b/tests/test_examples.py
index c7d3f29..926696c 100644
--- a/tests/test_examples.py
+++ b/tests/test_examples.py
@@ -20,7 +20,7 @@ def test_readme(self):
         male_teachers_with_children = OWLObjectIntersectionOf([males_with_children, teacher])
 
         assert owl_expression_to_dl(male_teachers_with_children)=="(∃ hasChild.male) ⊓ teacher"
-        assert owl_expression_to_sparql("?x", male_teachers_with_children)=="""SELECT
+        assert owl_expression_to_sparql(male_teachers_with_children)=="""SELECT
  DISTINCT ?x WHERE { 
 ?x <http://example.com/society#hasChild> ?s_1 . 
 ?s_1 a <http://example.com/society#male> .