diff --git a/examples/quality_of_owl_ce_via_sparql.py b/examples/quality_of_owl_ce_via_sparql.py
new file mode 100644
index 00000000..ea9603f5
--- /dev/null
+++ b/examples/quality_of_owl_ce_via_sparql.py
@@ -0,0 +1,23 @@
+"""
+The confusion matrix indicating the quality of an OWL Class Expression can be computed through SPARQL.
+By this, we avoid the process of retrieving instances of an OWL Class Expression, hence, accelerate the learning process
+"""
+from owlapy import owl_expression_to_sparql_with_confusion_matrix
+from owlapy.owl_individual import OWLNamedIndividual
+from owlapy.class_expression import OWLClass
+import requests
+
+pos={OWLNamedIndividual('http://dbpedia.org/resource/George_Montagu_(naturalist)'), OWLNamedIndividual('http://dbpedia.org/resource/Andrei_Monin'), OWLNamedIndividual('http://dbpedia.org/resource/Joe_Bastardi')}
+neg={OWLNamedIndividual('http://dbpedia.org/resource/James_M._Bower'), OWLNamedIndividual('http://dbpedia.org/resource/Shirley_Meng'), OWLNamedIndividual('http://dbpedia.org/resource/Betsy_Weatherhead')}
+response = requests.post("https://dbpedia-2022-12.data.dice-research.org/sparql", data={"query": owl_expression_to_sparql_with_confusion_matrix(expression=OWLClass('http://dbpedia.org/ontology/Person'),positive_examples=pos,negative_examples=neg)})
+for res in response.json()["results"]["bindings"]:
+    for k,v in res.items():
+        print(k,eval(v["value"]))
+
+
+"""
+tp 3.0
+fn 0.0
+fp 3.0
+tn 0.0
+"""
\ No newline at end of file
diff --git a/owlapy/converter.py b/owlapy/converter.py
index 426e32dd..b6ebcaa1 100644
--- a/owlapy/converter.py
+++ b/owlapy/converter.py
@@ -211,7 +211,7 @@ def current_variable(self):
     # each overload of the method is responsible for processing a different type of class expressions (e.g., ⊔ or ⊓)
     @singledispatchmethod
     def process(self, ce: OWLClassExpression):
-        raise NotImplementedError(ce)
+        raise NotImplementedError(f"We cannot create SPARQL query based on the following owl class {ce}")
 
     # an overload of process function
     # this overload is responsible for handling single concepts (e.g., Brother)
@@ -607,6 +607,7 @@ def as_query(self,
                  count: bool = False,
                  values: Optional[Iterable[OWLNamedIndividual]] = None,
                  named_individuals: bool = False) -> str:
+        assert isinstance(ce,OWLClassExpression), f"ce must be an instance of OWLClassExpression. Currently {type(ce)}"
         # root variable: the variable that will be projected
         # ce: the class expression to be transformed to a SPARQL query
         # for_all_de_morgan: true -> ¬(∃r.¬C), false -> (∀r.C)