diff --git a/iidm/iidm-api/src/main/java/com/powsybl/iidm/network/Identifiable.java b/iidm/iidm-api/src/main/java/com/powsybl/iidm/network/Identifiable.java
index 3cdaa61386a..6c49faf8383 100644
--- a/iidm/iidm-api/src/main/java/com/powsybl/iidm/network/Identifiable.java
+++ b/iidm/iidm-api/src/main/java/com/powsybl/iidm/network/Identifiable.java
@@ -63,7 +63,7 @@ default Properties getProperties() {
     /**
      * Set property value associated to specified key.
      */
-    Object setProperty(String key, String value);
+    String setProperty(String key, String value);
 
     /**
      * Get properties key values.
diff --git a/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/AbstractIdentifiable.java b/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/AbstractIdentifiable.java
index dab082eb748..406ee194147 100644
--- a/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/AbstractIdentifiable.java
+++ b/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/AbstractIdentifiable.java
@@ -66,7 +66,7 @@ public String getProperty(String key, String defaultValue) {
     }
 
     @Override
-    public Object setProperty(String key, String value) {
+    public String setProperty(String key, String value) {
         return properties.put(key, value);
     }
 
diff --git a/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/VariantManagerImplTest.java b/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/VariantManagerImplTest.java
index ead9087249d..50c8c1219ec 100644
--- a/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/VariantManagerImplTest.java
+++ b/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/VariantManagerImplTest.java
@@ -72,7 +72,7 @@ public String getProperty(String key, String defaultValue) {
         }
 
         @Override
-        public Object setProperty(String key, String value) {
+        public String setProperty(String key, String value) {
             return null;
         }