diff --git a/piplicenses.py b/piplicenses.py
index 49e3147..5b5d8e7 100644
--- a/piplicenses.py
+++ b/piplicenses.py
@@ -166,10 +166,9 @@ def get_pkg_info(pkg):
         for key in METADATA_KEYS:
             pkg_info[key] = metadata.get(key, LICENSE_UNKNOWN)
 
-        classifiers = metadata.get_all("classifier")
-        if classifiers:
-            pkg_info['license_classifier'] = \
-                find_license_from_classifier(classifiers)
+        classifiers = metadata.get_all("classifier", [])
+        pkg_info['license_classifier'] = \
+            find_license_from_classifier(classifiers)
 
         if args.filter_strings:
             for k in pkg_info:
@@ -437,13 +436,10 @@ def find_license_from_classifier(classifiers):
 
 
 def select_license_by_source(from_source, license_classifier, license_meta):
-    if isinstance(license_classifier, str):
-        license_classifier = [license_classifier]
     license_classifier_set = set(license_classifier) or {LICENSE_UNKNOWN}
     if (from_source == FromArg.CLASSIFIER or
             from_source == FromArg.MIXED and
-            len(license_classifier) > 0 and
-            LICENSE_UNKNOWN not in license_classifier):
+            len(license_classifier) > 0):
         return license_classifier_set
     else:
         return {license_meta}
diff --git a/test_piplicenses.py b/test_piplicenses.py
index 1723368..8bbaa96 100644
--- a/test_piplicenses.py
+++ b/test_piplicenses.py
@@ -239,7 +239,7 @@ def test_select_license_by_source(self):
                                                   'MIT'))
         self.assertEqual({'Apache License 2.0'},
                          select_license_by_source(FromArg.MIXED,
-                                                  'Apache License 2.0',
+                                                  ['Apache License 2.0'],
                                                   'Apache-2.0'))
 
     def test_with_system(self):