diff --git a/sket/rdf_proc/rdf_processing.py b/sket/rdf_proc/rdf_processing.py
index e6ba651..195ae60 100644
--- a/sket/rdf_proc/rdf_processing.py
+++ b/sket/rdf_proc/rdf_processing.py
@@ -621,7 +621,7 @@ def create_graph(self, rid, report_data, report_concepts, onto_proc, use_case, d
 		struct_graph = {}
 
 		# generate report id
-		hrid = 'r_' + rid
+		hrid = 'r_' + str(rid)
 		struct_graph['ReportID'] = hrid
 
 		# create report data-related triples
@@ -651,7 +651,7 @@ def create_graph(self, rid, report_data, report_concepts, onto_proc, use_case, d
 		struct_graph['patient'] = {}
 
 		# generate patient id
-		pid = 'p_' + rid
+		pid = 'p_' + str(rid)
 		# build the IRI for the patient
 		patient = self.base_iri + 'resource/patient/' + pid
 		struct_graph['patient']['PatientURL'] = patient