diff --git a/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/api/ISourceDirectoryContributor.kt b/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/api/ISourceDirectoryContributor.kt
index 91ea96a0..e8b25ab5 100644
--- a/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/api/ISourceDirectoryContributor.kt
+++ b/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/api/ISourceDirectoryContributor.kt
@@ -9,9 +9,12 @@ interface ISourceDirectoryContributor {
     fun sourceDirectoriesFor(project: Project, context: KobaltContext): List<File>
 }
 
-fun KobaltContext.sourceDirectories(project: Project) : List<File> {
+/**
+ * @return the source directories for this project including source contributors.
+ */
+fun KobaltContext.sourceDirectories(project: Project) : Set<File> {
     val result = pluginInfo.sourceDirContributors.flatMap {
         it.sourceDirectoriesFor(project, this)
     }
-    return result
+    return result.toSet()
 }
diff --git a/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/JvmCompilerPlugin.kt b/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/JvmCompilerPlugin.kt
index 7e6832f2..b2eab908 100644
--- a/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/JvmCompilerPlugin.kt
+++ b/modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/JvmCompilerPlugin.kt
@@ -144,8 +144,8 @@ open class JvmCompilerPlugin @Inject constructor(
 
     @IncrementalTask(name = JvmCompilerPlugin.TASK_COMPILE, description = "Compile the project")
     fun taskCompile(project: Project): IncrementalTaskInfo {
-        val inputChecksum = Md5.toMd5Directories(project.sourceDirectories.map {
-            File(project.directory, it)
+        val inputChecksum = Md5.toMd5Directories(context.sourceDirectories(project).map {
+            File(project.directory, it.path)
         })
         return IncrementalTaskInfo(
                 inputChecksum = inputChecksum,