diff --git a/service/lib/agama/storage/config_conversions/to_json_conversions/size.rb b/service/lib/agama/storage/config_conversions/to_json_conversions/size.rb
index 2a266f782c..8e110f9193 100644
--- a/service/lib/agama/storage/config_conversions/to_json_conversions/size.rb
+++ b/service/lib/agama/storage/config_conversions/to_json_conversions/size.rb
@@ -45,17 +45,25 @@ def convert
           # @see Base#conversions
           def conversions
             {
-              min: config.min&.to_i,
+              min: convert_min_size,
               max: convert_max_size
             }
           end
 
-          # @return [Integer, nil]
+          # @return [String, Integer]
+          def convert_min_size
+            return "current" unless config.min
+
+            config.min.to_i
+          end
+
+          # @return [String, Integer, nil]
           def convert_max_size
-            max = config.max
-            return if max.nil? || max.unlimited?
+            return "current" unless config.max
+
+            return if config.max.unlimited?
 
-            max.to_i
+            config.max.to_i
           end
         end
       end
diff --git a/service/package/rubygem-agama-yast.changes b/service/package/rubygem-agama-yast.changes
index e5596c0523..65efeec8d8 100644
--- a/service/package/rubygem-agama-yast.changes
+++ b/service/package/rubygem-agama-yast.changes
@@ -1,3 +1,8 @@
+-------------------------------------------------------------------
+Mon Sep 15 13:50:54 UTC 2025 - José Iván López González <jlopez@suse.com>
+
+- Fix JSON conversion for not configured sizes (bsc#1249591).
+
 -------------------------------------------------------------------
 Fri Sep 12 08:14:28 UTC 2025 - Ladislav Slezák <lslezak@suse.com>
 
diff --git a/service/test/agama/storage/config_conversions/to_json_conversions/examples.rb b/service/test/agama/storage/config_conversions/to_json_conversions/examples.rb
index c02eee9116..d3d9cbe385 100644
--- a/service/test/agama/storage/config_conversions/to_json_conversions/examples.rb
+++ b/service/test/agama/storage/config_conversions/to_json_conversions/examples.rb
@@ -369,6 +369,29 @@
       )
     end
 
+    context "if min size is not configured" do
+      let(:size) do
+        {
+          min: "1 GiB",
+          max: "10 GiB"
+        }
+      end
+
+      before do
+        config.size.min = nil
+      end
+
+      it "generates the expected JSON" do
+        config_json = subject.convert
+        expect(config_json[:size]).to eq(
+          {
+            min: "current",
+            max: 10.GiB.to_i
+          }
+        )
+      end
+    end
+
     context "if max size is unlimited" do
       let(:size) do
         {
@@ -386,6 +409,28 @@
       end
     end
 
+    context "if max size is not configured" do
+      let(:size) do
+        {
+          min: "1 GiB"
+        }
+      end
+
+      before do
+        config.size.max = nil
+      end
+
+      it "generates the expected JSON" do
+        config_json = subject.convert
+        expect(config_json[:size]).to eq(
+          {
+            min: 1.GiB.to_i,
+            max: "current"
+          }
+        )
+      end
+    end
+
     context "if size is default" do
       before do
         size_config = config.size