diff --git a/library/src/scala/collection/mutable/StringBuilder.scala b/library/src/scala/collection/mutable/StringBuilder.scala
index c283066eb64f..9d6077ebf604 100644
--- a/library/src/scala/collection/mutable/StringBuilder.scala
+++ b/library/src/scala/collection/mutable/StringBuilder.scala
@@ -109,7 +109,7 @@ final class StringBuilder(val underlying: java.lang.StringBuilder) extends Abstr
 
   def result() = underlying.toString
 
-  override def toString: String = result()
+  override def toString(): String = result()
 
   override def toArray[B >: Char](implicit ct: scala.reflect.ClassTag[B]) =
     ct.runtimeClass match {
diff --git a/tests/pos/i19616.scala b/tests/pos/i19616.scala
new file mode 100644
index 000000000000..2882b0b61e58
--- /dev/null
+++ b/tests/pos/i19616.scala
@@ -0,0 +1,6 @@
+def test: Unit =
+  val sb = new StringBuilder
+  var key = ""
+  var map = Map.empty[String, String]
+  map += key -> sb.toString()
+  map += key -> sb.toString