diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7052cf78..76325072 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,7 +12,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * Added
   * None
 * Fixed
-  * None
+  * [#726](https://github.com/binarylogic/authlogic/issues/726) - Thread
+    safety in `Authlogic::Session::Base.klass_name`
 
 ## 6.2.0 (2020-09-03)
 
diff --git a/lib/authlogic/session/base.rb b/lib/authlogic/session/base.rb
index e4b101a0..3fa6fc8e 100644
--- a/lib/authlogic/session/base.rb
+++ b/lib/authlogic/session/base.rb
@@ -776,15 +776,23 @@ def i18n_scope
         # example, the UserSession class will authenticate with the User class
         # unless you specify otherwise in your configuration. See
         # authenticate_with for information on how to change this value.
+        #
+        # @api public
         def klass
           @klass ||= klass_name ? klass_name.constantize : nil
         end
 
-        # The string of the model name class guessed from the actual session class name.
+        # The model name, guessed from the session class name, e.g. "User",
+        # from "UserSession".
+        #
+        # TODO: This method can return nil. We should explore this. It seems
+        # likely to cause a NoMethodError later, so perhaps we should raise an
+        # error instead.
+        #
+        # @api private
         def klass_name
-          return @klass_name if defined?(@klass_name)
-          @klass_name = name.scan(/(.*)Session/)[0]
-          @klass_name = klass_name ? klass_name[0] : nil
+          return @klass_name if instance_variable_defined?(:@klass_name)
+          @klass_name = name.scan(/(.*)Session/)[0]&.first
         end
 
         # The name of the method you want Authlogic to create for storing the