diff --git a/main/src/main/scala/sbt/APIMappings.scala b/main/src/main/scala/sbt/APIMappings.scala
index 7aa7f9632..1f7e4e9cc 100644
--- a/main/src/main/scala/sbt/APIMappings.scala
+++ b/main/src/main/scala/sbt/APIMappings.scala
@@ -22,7 +22,7 @@ private[sbt] object APIMappings
 
 	private[this] def parseURL(s: String, forEntry: File, log: Logger): Option[URL] =
 		try Some(new URL(s)) catch { case e: MalformedURLException =>
-			log.warn("Invalid API base URL '$s' for classpath entry '$forEntry': ${e.toString}")
+			log.warn(s"Invalid API base URL '$s' for classpath entry '$forEntry': ${e.toString}")
 			None
 		}