diff --git a/keeper-gradle-plugin/src/main/java/com/slack/keeper/ZipFlingerExt.kt b/keeper-gradle-plugin/src/main/java/com/slack/keeper/ZipFlingerExt.kt
index 1b1c4a2..672b4ce 100644
--- a/keeper-gradle-plugin/src/main/java/com/slack/keeper/ZipFlingerExt.kt
+++ b/keeper-gradle-plugin/src/main/java/com/slack/keeper/ZipFlingerExt.kt
@@ -30,7 +30,18 @@ internal fun File.classesSequence(): Sequence<Pair<String, File>> {
     .filter { it.extension == "class" }
     .filterNot { "META-INF" in it.name }
     .sortedBy { it.invariantSeparatorsPath }
-    .map { it.absolutePath.removePrefix(prefix).removePrefix("/") to it }
+    .map {
+      // zip specification "4.4.17.1 file name: (Variable)" items:
+      it.absolutePath
+        // "The name of the file, with optional relative path.
+        //  The path stored MUST NOT contain a drive or
+        //  device letter, or a leading slash"
+        .removePrefix(prefix)
+        .removePrefix(File.separator)
+        // "All slashes MUST be forward slashes '/' as opposed
+        // to backwards slashes '\' for compatibility"
+        .replace(File.separator, "/") to it
+    }
 }
 
 /** Extracts classes from the target [jar] into this archive. */