diff --git a/src/main/java/com/cleanroommc/groovyscript/sandbox/FileUtil.java b/src/main/java/com/cleanroommc/groovyscript/sandbox/FileUtil.java
index c1b39c494..891ed243e 100644
--- a/src/main/java/com/cleanroommc/groovyscript/sandbox/FileUtil.java
+++ b/src/main/java/com/cleanroommc/groovyscript/sandbox/FileUtil.java
@@ -20,6 +20,10 @@ public static String relativize(String rootPath, String longerThanRootPath) {
         if (File.separatorChar != '/') {
             longerThanRootPath = longerThanRootPath.replace('/', File.separatorChar);
         }
+        return relativizeInternal(fixDriveCase(rootPath), fixDriveCase(longerThanRootPath));
+    }
+
+    private static String relativizeInternal(String rootPath, String longerThanRootPath) {
         int index = longerThanRootPath.indexOf(rootPath);
         if (index < 0) {
             throw new IllegalArgumentException("The path '" + longerThanRootPath + "' does not contain the root path '" + rootPath + "'");
@@ -27,6 +31,15 @@ public static String relativize(String rootPath, String longerThanRootPath) {
         return longerThanRootPath.substring(index + rootPath.length() + 1);
     }
 
+    // sometimes the paths passed to relativize() have a lower case drive letter
+    public static String fixDriveCase(String path) {
+        if (path == null || path.length() < 2) return path;
+        if (Character.isLowerCase(path.charAt(0)) && path.charAt(1) == ':') {
+            return Character.toUpperCase(path.charAt(0)) + ':' + path.substring(2);
+        }
+        return path;
+    }
+
     public static String getParent(String path) {
         int i = path.lastIndexOf(File.separatorChar);
         if (i <= 0) return StringUtils.EMPTY;