diff --git a/build.gradle b/build.gradle
index dabc0367f..b0173f5de 100755
--- a/build.gradle
+++ b/build.gradle
@@ -103,7 +103,7 @@ javadoc {
     options.addStringOption('Xwerror', '-quiet') // makes the build fail on travis when there is a javadoc error
     options.linkSource true
     options.encoding "UTF-8" // allow emoji in comments :^)
-    source += sourceSets.api.allJava
+    source = sourceSets.api.allJava
     classpath += sourceSets.api.compileClasspath
 }