diff --git a/logic_test.ts b/logic_test.ts
index be980f7..cd15702 100644
--- a/logic_test.ts
+++ b/logic_test.ts
@@ -72,6 +72,10 @@ Deno.test("getRemovableFiles never removes #keeplist.txt", () => {
 Deno.test("resolveKeeplistName uses default and prefixed keeplist names deterministically", () => {
   assertEquals(resolveKeeplistName(null), "#keeplist.txt");
   assertEquals(resolveKeeplistName("vanilla"), "#vanilla-keeplist.txt");
+  assertEquals(
+    resolveKeeplistName("Vanilla_Profile"),
+    "#Vanilla_Profile-keeplist.txt",
+  );
 });
 
 Deno.test("getRemovableFiles never removes the active prefixed keeplist", () => {
diff --git a/main.ts b/main.ts
index 9ebc5f9..f504547 100644
--- a/main.ts
+++ b/main.ts
@@ -490,14 +490,13 @@ function applyPrefixValue(rawValue) {
     return true;
   }
 
-  const normalized = trimmed.toLowerCase();
-  if (!/^[a-z0-9_-]+$/.test(normalized)) {
+  if (!/^[A-Za-z0-9_-]+$/.test(trimmed)) {
     setPrefixValidationError("Prefix must use only letters, numbers, underscores, or hyphens.");
     updateActiveKeeplistName();
     return false;
   }
 
-  keeplistPrefix = normalized;
+  keeplistPrefix = trimmed;
   setPrefixValidationError("");
   updateActiveKeeplistName();
   return true;