diff --git a/src/main/org/apache/tools/ant/Main.java b/src/main/org/apache/tools/ant/Main.java
index 98b5cf4c0..9334dab5a 100644
--- a/src/main/org/apache/tools/ant/Main.java
+++ b/src/main/org/apache/tools/ant/Main.java
@@ -378,6 +378,9 @@ public class Main implements AntMain {
                     name = name.substring(0, posEq);
                 } else if (i < args.length - 1) {
                     value = args[++i];
+                } else {
+                    throw new BuildException("Missing value for property "
+                                             + name);
                 }
 
                 definedProps.put(name, value);