diff --git a/gns3server/schemas/dynamips_vm.py b/gns3server/schemas/dynamips_vm.py
index ee4c8883..2ab0f20a 100644
--- a/gns3server/schemas/dynamips_vm.py
+++ b/gns3server/schemas/dynamips_vm.py
@@ -59,7 +59,7 @@ VM_CREATE_SCHEMA = {
         },
         "image_md5sum": {
             "description": "checksum of the IOS image",
-            "type": "string",
+            "type": ["string", "null"],
             "minLength": 1,
         },
         "startup_config": {
@@ -295,7 +295,7 @@ VM_UPDATE_SCHEMA = {
         },
         "image_md5sum": {
             "description": "checksum of the IOS image",
-            "type": "string",
+            "type": ["string", "null"],
             "minLength": 1,
         },
         "startup_config_content": {
@@ -558,7 +558,7 @@ VM_OBJECT_SCHEMA = {
         },
         "image_md5sum": {
             "description": "checksum of the IOS image",
-            "type": "string",
+            "type": ["string", "null"],
             "minLength": 1,
         },
         "startup_config": {
diff --git a/gns3server/schemas/iou.py b/gns3server/schemas/iou.py
index 63c11165..019d0dc4 100644
--- a/gns3server/schemas/iou.py
+++ b/gns3server/schemas/iou.py
@@ -48,7 +48,7 @@ IOU_CREATE_SCHEMA = {
         },
         "md5sum": {
             "description": "Checksum of iou binary",
-            "type": "string"
+            "type": ["string", "null"]
         },
         "serial_adapters": {
             "description": "How many serial adapters are connected to the IOU",
@@ -121,7 +121,7 @@ IOU_UPDATE_SCHEMA = {
         },
         "md5sum": {
             "description": "Checksum of iou binary",
-            "type": "string"
+            "type": ["string", "null"]
         },
         "serial_adapters": {
             "description": "How many serial adapters are connected to the IOU",
@@ -199,7 +199,7 @@ IOU_OBJECT_SCHEMA = {
         },
         "md5sum": {
             "description": "Checksum of iou binary",
-            "type": "string"
+            "type": ["string", "null"]
         },
         "serial_adapters": {
             "description": "How many serial adapters are connected to the IOU",