diff --git a/src/micropolisj/engine/ToolEffect.java b/src/micropolisj/engine/ToolEffect.java
index 2078e76..c883d3f 100644
--- a/src/micropolisj/engine/ToolEffect.java
+++ b/src/micropolisj/engine/ToolEffect.java
@@ -42,7 +42,9 @@ class ToolEffect implements ToolEffectIfc
 			return city.getTile(originX + dx, originY + dy);
 		}
 		else {
-			return CLEAR;
+			// tiles outside city's boundary assumed to be
+			// tile #0 (dirt).
+			return 0;
 		}
 	}
 
diff --git a/src/micropolisj/engine/ToolEffectIfc.java b/src/micropolisj/engine/ToolEffectIfc.java
index 11e1c86..aac4688 100644
--- a/src/micropolisj/engine/ToolEffectIfc.java
+++ b/src/micropolisj/engine/ToolEffectIfc.java
@@ -12,6 +12,7 @@ public interface ToolEffectIfc
 {
 	/**
 	 * Gets the tile at a relative location.
+	 * @return a non-negative tile identifier
 	 */
 	int getTile(int dx, int dy);