diff --git a/src/main/java/org/spdx/tools/schema/OwlToJsonContext.java b/src/main/java/org/spdx/tools/schema/OwlToJsonContext.java
index ad5a9fe..ab0ec77 100644
--- a/src/main/java/org/spdx/tools/schema/OwlToJsonContext.java
+++ b/src/main/java/org/spdx/tools/schema/OwlToJsonContext.java
@@ -82,10 +82,12 @@ public ObjectNode convertToContext() {
 		ExtendedIterator<OntProperty> iter = model.listAllOntProperties();
 		while (iter.hasNext()) {
 			OntProperty property = iter.next();
-			String propNamespace = uriToNamespace(property.getURI());
-			String propName = uriToPropName(property.getURI());
-			String id = propNamespace + propName;
-			sortedOntProperties.put(id, property);
+			if (property.isURIResource()) {
+				String propNamespace = uriToNamespace(property.getURI());
+				String propName = uriToPropName(property.getURI());
+				String id = propNamespace + propName;
+				sortedOntProperties.put(id, property);
+			}
 		}
 		for (Entry<String, OntProperty> ontPropEntry:sortedOntProperties.entrySet()) {
 			String propNamespace = uriToNamespace(ontPropEntry.getValue().getURI());
diff --git a/src/main/java/org/spdx/tools/schema/OwlToJsonSchema.java b/src/main/java/org/spdx/tools/schema/OwlToJsonSchema.java
index cf6f91b..ea0ff9f 100644
--- a/src/main/java/org/spdx/tools/schema/OwlToJsonSchema.java
+++ b/src/main/java/org/spdx/tools/schema/OwlToJsonSchema.java
@@ -93,14 +93,16 @@ public ObjectNode convertToJsonSchema() {
 		ExtendedIterator<Ontology> ontologyIter = model.listOntologies();
 		if (ontologyIter.hasNext()) {
 			Ontology ont = ontologyIter.next();
-			String version = ont.getVersionInfo();
-			String ontologyUri = version == null ? ont.getURI() : ont.getURI() + "/" + version;
-			if (Objects.nonNull(ontologyUri)) {
-				root.put("$id", ontologyUri);
-			}
-			String title = ont.getLabel(null);
-			if (Objects.nonNull(title)) {
-				root.put("title", title);
+			if (ont.isURIResource()) {
+				String version = ont.getVersionInfo();
+				String ontologyUri = version == null ? ont.getURI() : ont.getURI() + "/" + version;
+				if (Objects.nonNull(ontologyUri)) {
+					root.put("$id", ontologyUri);
+				}
+				String title = ont.getLabel(null);
+				if (Objects.nonNull(title)) {
+					root.put("title", title);
+				}
 			}
 		}
 		root.put(JSON_RESTRICTION_TYPE,JSON_TYPE_OBJECT);
diff --git a/src/main/java/org/spdx/tools/schema/OwlToXsd.java b/src/main/java/org/spdx/tools/schema/OwlToXsd.java
index 4fdbfa1..15815f8 100644
--- a/src/main/java/org/spdx/tools/schema/OwlToXsd.java
+++ b/src/main/java/org/spdx/tools/schema/OwlToXsd.java
@@ -71,6 +71,9 @@ public OwlToXsd(OntModel model) {
 	}
 	
 	public XmlSchema convertToXsd() throws XmlSchemaSerializerException, SchemaException {
+		if (!ontology.isURIResource()) {
+			throw new SchemaException("Ontology is not a URI resource");
+		}
 		String nameSpace = ontology.getURI();
 		XmlSchema schema = new XmlSchema(nameSpace, schemas);
 		schema.setSchemaNamespacePrefix(nameSpace);