diff --git a/scripts/generate_object_properties.cr b/scripts/generate_object_properties.cr
index ced3e7d69028..2f809ec45235 100755
--- a/scripts/generate_object_properties.cr
+++ b/scripts/generate_object_properties.cr
@@ -35,20 +35,46 @@ struct Generator
   end
 
   def def_getter(suffix = "")
+    if @macro_prefix == "class_"
+      def_class_getter suffix
+    else
+      <<-TEXT
+            def {{var_name}}#{suffix} {% if type %} : {{type}} {% end %}
+              {% if block %}
+                if (%value = @{{var_name}}).nil?
+                  @{{var_name}} = {{yield}}
+                else
+                  %value
+                end
+              {% else %}
+                @{{var_name}}
+              {% end %}
+            end
+
+      TEXT
+    end
+  end
+
+  def def_class_getter(suffix = "")
     <<-TEXT
-          def #{@method_prefix}{{var_name}}#{suffix} {% if type %} : {{type}} {% end %}
-            {% if block %}
-              if (%value = #{@var_prefix}{{var_name}}).nil?
-                #{@var_prefix}{{var_name}} = {{yield}}
-              else
-                %value
-              end
-            {% else %}
-              #{@var_prefix}{{var_name}}
-            {% end %}
-          end
+            {% if block %} @@__{{var_name}}_flag = false {% end %}
+
+            def self.{{var_name}}#{suffix} {% if type %} : {{type}} {% end %}
+              {% if block %}
+                if (%value = @@{{var_name}}).nil?
+                  ::Crystal.once(pointerof(@@__{{var_name}}_flag)) do
+                    @@{{var_name}} = {{yield}} if @@{{var_name}}.nil?
+                  end
+                  @@{{var_name}}.not_nil!
+                else
+                  %value
+                end
+              {% else %}
+                @@{{var_name}}
+              {% end %}
+            end
 
-    TEXT
+      TEXT
   end
 
   def def_vars!
diff --git a/src/object/properties.cr b/src/object/properties.cr
index c9bc1c74c4c1..fe34eec415e2 100644
--- a/src/object/properties.cr
+++ b/src/object/properties.cr
@@ -993,10 +993,15 @@ class Object
         {% type = nil %}
       {% end %}
 
+      {% if block %} @@__{{var_name}}_flag = false {% end %}
+
       def self.{{var_name}} {% if type %} : {{type}} {% end %}
         {% if block %}
           if (%value = @@{{var_name}}).nil?
-            @@{{var_name}} = {{yield}}
+            ::Crystal.once(pointerof(@@__{{var_name}}_flag)) do
+              @@{{var_name}} = {{yield}} if @@{{var_name}}.nil?
+            end
+            @@{{var_name}}.not_nil!
           else
             %value
           end
@@ -1120,10 +1125,15 @@ class Object
         {% type = nil %}
       {% end %}
 
+      {% if block %} @@__{{var_name}}_flag = false {% end %}
+
       def self.{{var_name}}? {% if type %} : {{type}} {% end %}
         {% if block %}
           if (%value = @@{{var_name}}).nil?
-            @@{{var_name}} = {{yield}}
+            ::Crystal.once(pointerof(@@__{{var_name}}_flag)) do
+              @@{{var_name}} = {{yield}} if @@{{var_name}}.nil?
+            end
+            @@{{var_name}}.not_nil!
           else
             %value
           end
@@ -1469,10 +1479,15 @@ class Object
         {% type = nil %}
       {% end %}
 
+      {% if block %} @@__{{var_name}}_flag = false {% end %}
+
       def self.{{var_name}} {% if type %} : {{type}} {% end %}
         {% if block %}
           if (%value = @@{{var_name}}).nil?
-            @@{{var_name}} = {{yield}}
+            ::Crystal.once(pointerof(@@__{{var_name}}_flag)) do
+              @@{{var_name}} = {{yield}} if @@{{var_name}}.nil?
+            end
+            @@{{var_name}}.not_nil!
           else
             %value
           end
@@ -1611,10 +1626,15 @@ class Object
         {% type = nil %}
       {% end %}
 
+      {% if block %} @@__{{var_name}}_flag = false {% end %}
+
       def self.{{var_name}}? {% if type %} : {{type}} {% end %}
         {% if block %}
           if (%value = @@{{var_name}}).nil?
-            @@{{var_name}} = {{yield}}
+            ::Crystal.once(pointerof(@@__{{var_name}}_flag)) do
+              @@{{var_name}} = {{yield}} if @@{{var_name}}.nil?
+            end
+            @@{{var_name}}.not_nil!
           else
             %value
           end