diff --git a/src/main/org/apache/tools/ant/taskdefs/Property.java b/src/main/org/apache/tools/ant/taskdefs/Property.java
index f84348247..6725b02b2 100644
--- a/src/main/org/apache/tools/ant/taskdefs/Property.java
+++ b/src/main/org/apache/tools/ant/taskdefs/Property.java
@@ -279,7 +279,7 @@ public class Property extends Task {
      */
     public void setPrefix(String prefix) {
         this.prefix = prefix;
-        if (!prefix.endsWith(".")) {
+        if (prefix != null && !prefix.endsWith(".")) {
             this.prefix += ".";
         }
     }