diff --git a/scm-ui-components/packages/ui-components/src/errors.js b/scm-ui-components/packages/ui-components/src/errors.js
index 13e4996cf7..5b11eab0b8 100644
--- a/scm-ui-components/packages/ui-components/src/errors.js
+++ b/scm-ui-components/packages/ui-components/src/errors.js
@@ -52,6 +52,13 @@ export class NotFoundError extends BackendError {
     super(content, "NotFoundError", statusCode);
   }
 }
+
+export class ConflictError extends BackendError {
+  constructor(content: BackendErrorContent, statusCode: number) {
+    super(content, "ConflictError", statusCode);
+  }
+}
+
 export function createBackendError(
   content: BackendErrorContent,
   statusCode: number
@@ -59,6 +66,8 @@ export function createBackendError(
   switch (statusCode) {
     case 404:
       return new NotFoundError(content, statusCode);
+    case 409:
+      return new ConflictError(content, statusCode);
     default:
       return new BackendError(content, "BackendError", statusCode);
   }
diff --git a/scm-webapp/src/main/java/sonia/scm/api/v2/resources/MergeResource.java b/scm-webapp/src/main/java/sonia/scm/api/v2/resources/MergeResource.java
index 63fa2274ec..2214b258c6 100644
--- a/scm-webapp/src/main/java/sonia/scm/api/v2/resources/MergeResource.java
+++ b/scm-webapp/src/main/java/sonia/scm/api/v2/resources/MergeResource.java
@@ -4,6 +4,7 @@ import com.webcohesion.enunciate.metadata.rs.ResponseCode;
 import com.webcohesion.enunciate.metadata.rs.StatusCodes;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.http.HttpStatus;
+import sonia.scm.ConcurrentModificationException;
 import sonia.scm.repository.NamespaceAndName;
 import sonia.scm.repository.api.MergeCommandBuilder;
 import sonia.scm.repository.api.MergeCommandResult;
@@ -73,7 +74,7 @@ public class MergeResource {
       if (mergeCommandResult.isMergeable()) {
         return Response.noContent().build();
       } else {
-        return Response.status(HttpStatus.SC_CONFLICT).build();
+        throw new ConcurrentModificationException("revision", mergeCommand.getTargetRevision());
       }
     }
   }