diff --git a/project/Build.scala b/project/Build.scala
index 09feed9..a006dd3 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -9,7 +9,7 @@ object SbtIdeaBuild extends Build with BuildExtra {
     organization := "com.github.mpeltonen",
     name := "sbt-idea",
     version := "1.7.0-SNAPSHOT",
-    sbtVersion in Global := "0.13.2",
+    sbtVersion in Global := "0.13.5",
     scalaVersion in Global := "2.10.3",
     publishTo <<= version { (v: String) =>
       val nexus = "https://oss.sonatype.org/"
diff --git a/src/main/scala/org/sbtidea/SbtIdeaPlugin.scala b/src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
index 4e975a4..37c134b 100644
--- a/src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
+++ b/src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
@@ -203,7 +203,7 @@ object SbtIdeaPlugin extends Plugin {
     val compileDirectories: Directories = directoriesFor(Configurations.Compile)
 
     def appendExtraTestDirectories(directories: Directories) = {
-      val extraConfigurations = (Seq(Configurations.IntegrationTest) ++ settings.setting(ideaExtraTestConfigurations, "Missing extra test configuration"))
+      val extraConfigurations = (Seq(Configurations.IntegrationTest) ++ settings.settingWithDefault(ideaExtraTestConfigurations, Seq()))
       extraConfigurations.foldLeft(directories)((dirs,conf) =>
         dirs.addSrc(sourceDirectoriesFor(conf)).addRes(resourceDirectoriesFor(conf))
       )
@@ -211,11 +211,11 @@ object SbtIdeaPlugin extends Plugin {
     val testDirectories: Directories = appendExtraTestDirectories(directoriesFor(Configurations.Test))
     val librariesExtractor = new SbtIdeaModuleMapping.LibrariesExtractor(buildStruct, state, projectRef, scalaInstance,
       withClassifiers = if (args.contains(NoClassifiers)) None else {
-        Some((settings.setting(ideaSourcesClassifiers, "Missing idea-sources-classifiers"), settings.setting(ideaJavadocsClassifiers, "Missing idea-javadocs-classifiers")))
+        Some((settings.settingWithDefault(ideaSourcesClassifiers, Seq("sources")), settings.settingWithDefault(ideaJavadocsClassifiers, Seq("javadoc"))))
       }
     )
-    val basePackage = settings.setting(ideaBasePackage, "missing IDEA base package")
-    val packagePrefix = settings.setting(ideaPackagePrefix, "missing package prefix")
+    val basePackage = settings.optionalSetting(ideaBasePackage).flatten
+    val packagePrefix = settings.optionalSetting(ideaPackagePrefix).flatten
     val extraFacets = settings.settingWithDefault(ideaExtraFacets, NodeSeq.Empty)
     val includeScalaFacet = settings.settingWithDefault(ideaIncludeScalaFacet, true)
     def isAggregate(p: String) = allProjectIds(p)