Index: trunk/src/org/openstreetmap/josm/gui/widgets/JosmComboBox.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/widgets/JosmComboBox.java	(revision 9545)
+++ trunk/src/org/openstreetmap/josm/gui/widgets/JosmComboBox.java	(revision 9546)
@@ -4,4 +4,5 @@
 import java.awt.Component;
 import java.awt.Dimension;
+import java.awt.GraphicsEnvironment;
 import java.awt.Toolkit;
 import java.awt.event.MouseAdapter;
@@ -166,5 +167,5 @@
         if (prototype != null) {
             setPrototypeDisplayValue(prototype);
-            int screenHeight = Toolkit.getDefaultToolkit().getScreenSize().height;
+            int screenHeight = GraphicsEnvironment.isHeadless() ? 600 : Toolkit.getDefaultToolkit().getScreenSize().height;
             // Compute maximum number of visible items based on the preferred size of the combo box.
             // This assumes that items have the same height as the combo box, which is not granted by the look and feel