diff --git a/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCApi.java b/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCApi.java
index 7a8a4628c7..03ec4357cf 100644
--- a/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCApi.java
+++ b/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCApi.java
@@ -32,10 +32,12 @@ import org.jclouds.fujitsu.fgcp.domain.ServerType;
 import org.jclouds.fujitsu.fgcp.domain.UsageInfo;
 import org.jclouds.fujitsu.fgcp.domain.VSystem;
 import org.jclouds.fujitsu.fgcp.domain.VSystemDescriptor;
+import org.jclouds.javax.annotation.Nullable;
 
 /**
  * API relating to the virtual data center.
  * 
+ * @see VirtualDCAsyncApi
  * @author Dies Koper
  */
 @Timeout(duration = 60, timeUnit = TimeUnit.SECONDS)
@@ -51,8 +53,7 @@ public interface VirtualDCApi {
 
    Set<DiskImage> listDiskImages();
 
-   Set<DiskImage> listDiskImages(String serverCategory,
-         String vsysDescriptorId);
+   Set<DiskImage> listDiskImages(@Nullable String serverCategory, String vsysDescriptorId);
 
    Map<PublicIP, String> listPublicIPs();
 
diff --git a/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCAsyncApi.java b/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCAsyncApi.java
index b9e85191ad..7836893678 100644
--- a/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCAsyncApi.java
+++ b/labs/fgcp/src/main/java/org/jclouds/fujitsu/fgcp/services/VirtualDCAsyncApi.java
@@ -41,6 +41,7 @@ import org.jclouds.fujitsu.fgcp.domain.VSystem;
 import org.jclouds.fujitsu.fgcp.domain.VSystemDescriptor;
 import org.jclouds.fujitsu.fgcp.filters.RequestAuthenticator;
 import org.jclouds.fujitsu.fgcp.reference.RequestParameters;
+import org.jclouds.javax.annotation.Nullable;
 import org.jclouds.rest.annotations.JAXBResponseParser;
 import org.jclouds.rest.annotations.PayloadParams;
 import org.jclouds.rest.annotations.QueryParams;
@@ -102,7 +103,7 @@ public interface VirtualDCAsyncApi {
    @JAXBResponseParser
    @QueryParams(keys = "Action", values = "ListDiskImage")
    ListenableFuture<Set<DiskImage>> listDiskImages(
-         @QueryParam("serverCategory") String serverCategory,
+         @Nullable @QueryParam("serverCategory") String serverCategory,
          @QueryParam("vsysDescriptorId") String vsysDescriptorId);
 
    /**