diff --git a/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala b/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala
index e9a02baafd28e7f48cbe3b39391f4d2cff3d74fd..a2c4bc2f5480b17215c534b8571040e3a7858567 100644
--- a/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala
+++ b/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala
@@ -1045,7 +1045,10 @@ object Client extends Logging {
           s"in favor of the $CONF_SPARK_JAR configuration variable.")
       System.getenv(ENV_SPARK_JAR)
     } else {
-      SparkContext.jarOfClass(this.getClass).head
+      SparkContext.jarOfClass(this.getClass).getOrElse(throw new SparkException("Could not "
+        + "find jar containing Spark classes. The jar can be defined using the "
+        + "spark.yarn.jar configuration option. If testing Spark, either set that option or "
+        + "make sure SPARK_PREPEND_CLASSES is not set."))
     }
   }