diff --git a/io/src/main/scala/sbt/io/IO.scala b/io/src/main/scala/sbt/io/IO.scala
index bc1245ce..932148b0 100644
--- a/io/src/main/scala/sbt/io/IO.scala
+++ b/io/src/main/scala/sbt/io/IO.scala
@@ -608,8 +608,8 @@ object IO {
    * If `file` or `base` are not absolute, they are first resolved against the current working directory.
    */
   def relativize(base: File, file: File): Option[String] = {
-    val basePath = base.toPath
-    val filePath = file.toPath
+    val basePath = (if (base.isAbsolute) base else base.getCanonicalFile).toPath
+    val filePath = (if (file.isAbsolute) file else file.getCanonicalFile).toPath
     if ((filePath startsWith basePath) || (filePath.normalize() startsWith basePath.normalize())) {
       val relativePath = catching(classOf[IllegalArgumentException]) opt (basePath relativize filePath)
       relativePath map (_.toString)
diff --git a/io/src/test/scala/sbt/io/IOSpec.scala b/io/src/test/scala/sbt/io/IOSpec.scala
index 00ed1a44..11621eb7 100644
--- a/io/src/test/scala/sbt/io/IOSpec.scala
+++ b/io/src/test/scala/sbt/io/IOSpec.scala
@@ -36,6 +36,12 @@ class IOSpec extends FlatSpec with Matchers {
     IO.relativize(base, file3) shouldBe Some(".git")
   }
 
+  it should "relativize relative paths" in {
+    val base = new File(".").getCanonicalFile
+    val file = new File("build.sbt")
+    IO.relativize(base, file) shouldBe Some("build.sbt")
+  }
+
   "toURI" should "make URI" in {
     val u = IO.toURI(file("/etc/hosts"))
     assert(u.toString == "file:///etc/hosts")