Index: /trunk/src/org/openstreetmap/josm/gui/mappaint/styleelement/LineElement.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/mappaint/styleelement/LineElement.java	(revision 13012)
+++ /trunk/src/org/openstreetmap/josm/gui/mappaint/styleelement/LineElement.java	(revision 13013)
@@ -357,4 +357,8 @@
         }
         float dashesOffset = c.get(type.prefix + DASHES_OFFSET, 0f, Float.class);
+        if (dashesOffset < 0f) {
+            Logging.warn("Found negative " + DASHES_OFFSET + ": " + dashesOffset);
+            dashesOffset = 0f;
+        }
         Color dashesBackground = c.get(type.prefix + DASHES_BACKGROUND_COLOR, null, Color.class);
         if (dashesBackground != null) {