diff --git a/source/rafcon/statemachine/states/library_state.py b/source/rafcon/statemachine/states/library_state.py
index <HASH>..<HASH> 100644
--- a/source/rafcon/statemachine/states/library_state.py
+++ b/source/rafcon/statemachine/states/library_state.py
@@ -326,7 +326,7 @@ class LibraryState(State):
     @version.setter
     @Observable.observed
     def version(self, version):
-        if not isinstance(version, basestring):
+        if version is not None and not isinstance(version, basestring):
             raise TypeError("version must be of type str")
 
         self._version = version
Allow 'None' as value for the library version