diff --git a/compile/api/src/main/scala/sbt/ClassToAPI.scala b/compile/api/src/main/scala/sbt/ClassToAPI.scala
index 37e8a5919..8511d3599 100644
--- a/compile/api/src/main/scala/sbt/ClassToAPI.scala
+++ b/compile/api/src/main/scala/sbt/ClassToAPI.scala
@@ -160,7 +160,7 @@ object ClassToAPI {
           } catch {
             case e: Throwable =>
               throw new IllegalStateException(
-                s"Failed to parse class $c: this may mean your classfiles are corrupted. Please clean and try again.",
+                s"Failed to parse $c: this may mean your classfiles are corrupted. Please clean and try again.",
                 e
               )
           }
diff --git a/util/io/src/main/scala/sbt/IO.scala b/util/io/src/main/scala/sbt/IO.scala
index 348fd0ab3..0fd5f52ad 100644
--- a/util/io/src/main/scala/sbt/IO.scala
+++ b/util/io/src/main/scala/sbt/IO.scala
@@ -38,14 +38,23 @@ object IO {
   /**
    * Returns a URL for the directory or jar containing the the class file `cl`.
    * If the location cannot be determined, an error is generated.
-   * Note that Java standard library classes typically do not have a location associated with them.
    */
-  def classLocation(cl: Class[_]): URL =
-    {
-      val codeSource = cl.getProtectionDomain.getCodeSource
-      if (codeSource == null) sys.error("No class location for " + cl)
-      else codeSource.getLocation
+  def classLocation(cl: Class[_]): URL = {
+    val codeSource = cl.getProtectionDomain.getCodeSource
+    if (codeSource ne null) {
+      codeSource.getLocation
+    } else {
+      // NB: This assumes that classes without code sources are System classes, and thus located in
+      // jars. It assumes that `urlAsFile` will truncate to the containing jar file.
+      val clsfile = s"${cl.getName.replace('.', '/')}.class"
+      Option(ClassLoader.getSystemClassLoader.getResource(clsfile))
+        .flatMap {
+          urlAsFile
+        }.getOrElse {
+          sys.error("No class location for " + cl)
+        }.toURI.toURL
     }
+  }
 
   /**
    * Returns the directory or jar file containing the the class file `cl`.
diff --git a/util/io/src/test/scala/CopySpec.scala b/util/io/src/test/scala/sbt/CopySpec.scala
similarity index 100%
rename from util/io/src/test/scala/CopySpec.scala
rename to util/io/src/test/scala/sbt/CopySpec.scala
diff --git a/util/io/src/test/scala/FileUtilitiesSpecification.scala b/util/io/src/test/scala/sbt/FileUtilitiesSpecification.scala
similarity index 100%
rename from util/io/src/test/scala/FileUtilitiesSpecification.scala
rename to util/io/src/test/scala/sbt/FileUtilitiesSpecification.scala
diff --git a/util/io/src/test/scala/sbt/IOSpecification.scala b/util/io/src/test/scala/sbt/IOSpecification.scala
new file mode 100644
index 000000000..882f147c5
--- /dev/null
+++ b/util/io/src/test/scala/sbt/IOSpecification.scala
@@ -0,0 +1,25 @@
+package sbt
+
+import util.Try
+
+import org.scalacheck._
+import Prop._
+
+object IOSpecification extends Properties("IO") {
+  property("classLocation able to determine containing directories") =
+    Prop.forAll(classes) { (c: Class[_]) =>
+      Try(IO.classLocationFile(c)).toOption.exists {
+        case jar if jar.getName.endsWith(".jar") => jar.isFile
+        case dir                                 => dir.isDirectory
+      }
+    }
+
+  implicit def classes: Gen[Class[_]] =
+    Gen.oneOf(
+      this.getClass,
+      classOf[java.lang.Integer],
+      classOf[String],
+      classOf[Thread],
+      classOf[Properties]
+    )
+}
diff --git a/util/io/src/test/scala/NameFilterSpecification.scala b/util/io/src/test/scala/sbt/NameFilterSpecification.scala
similarity index 100%
rename from util/io/src/test/scala/NameFilterSpecification.scala
rename to util/io/src/test/scala/sbt/NameFilterSpecification.scala
diff --git a/util/io/src/test/scala/RichURISpecification.scala b/util/io/src/test/scala/sbt/RichURISpecification.scala
similarity index 100%
rename from util/io/src/test/scala/RichURISpecification.scala
rename to util/io/src/test/scala/sbt/RichURISpecification.scala
diff --git a/util/io/src/test/scala/StashSpec.scala b/util/io/src/test/scala/sbt/StashSpec.scala
similarity index 100%
rename from util/io/src/test/scala/StashSpec.scala
rename to util/io/src/test/scala/sbt/StashSpec.scala
diff --git a/util/io/src/test/scala/WithFiles.scala b/util/io/src/test/scala/sbt/WithFiles.scala
similarity index 100%
rename from util/io/src/test/scala/WithFiles.scala
rename to util/io/src/test/scala/sbt/WithFiles.scala