diff --git a/funowl/ontology_document.py b/funowl/ontology_document.py
index a12a800..0729341 100644
--- a/funowl/ontology_document.py
+++ b/funowl/ontology_document.py
@@ -175,9 +175,10 @@ def objectPropertyRange(self, ope: ObjectPropertyExpression.types(), ce: ClassEx
         self.axioms.append(ObjectPropertyRange(ope, ce))
         return self
 
-    def imports(self, import_: Union["Ontology", str]) -> "Ontology":
+    def imports(self, import_: Union["Ontology", "OntologyDocument", str]) -> "Ontology":
         self.directlyImportsDocuments.append(
-            Import(import_.iri if isinstance(import_, Ontology) else IRI(str(import_))))
+            Import(import_.iri if isinstance(import_, Ontology) else
+                   import_.ontology.iri if isinstance(import_, OntologyDocument) else IRI(str(import_))))
         return self
 
     def namedIndividuals(self, *individuals: IRI.types()) -> "Ontology":
diff --git a/funowl/prefix_declarations.py b/funowl/prefix_declarations.py
index e13cb2c..804ff7d 100644
--- a/funowl/prefix_declarations.py
+++ b/funowl/prefix_declarations.py
@@ -1,10 +1,11 @@
 from dataclasses import dataclass
-from typing import Optional, List
+from typing import Optional, List, Union
 
 from rdflib import Graph
 from rdflib.namespace import NamespaceManager, OWL
 from rdflib.term import URIRef
 
+from funowl.base.cast_function import exclude
 from funowl.base.fun_owl_base import FunOwlBase
 from funowl.general_definitions import PrefixName, FullIRI
 from funowl.writers.FunctionalWriter import FunctionalWriter
@@ -12,9 +13,13 @@
 
 @dataclass
 class Prefix(FunOwlBase):
-    prefixName: Optional[PrefixName]
+    prefixName: Optional[Union[PrefixName, str]]
     fullIRI: FullIRI
 
+    def __post_init__(self):
+        if self.prefixName is not None and not isinstance(self.prefixName, PrefixName):
+            self.prefixName = PrefixName(self.prefixName)
+
     def to_functional(self, w: FunctionalWriter) -> FunctionalWriter:
         return w.func(self, lambda: w.concat((self.prefixName or '') + ':', '=',
                                              URIRef(str(self.fullIRI)).n3(), sep=' '))