diff --git a/src/krate.rs b/src/krate.rs
index 2638c15798..0a3247b04d 100644
--- a/src/krate.rs
+++ b/src/krate.rs
@@ -745,11 +745,6 @@ pub fn new(req: &mut Request) -> CargoResult<Response> {
     let ignored_invalid_categories = try!(
         Category::update_crate(try!(req.tx()), &krate, &categories)
     );
-    let warnings: Vec<String> =
-        ignored_invalid_categories.iter().map(|category| {
-            format!("'{}' is not a recognized category name \
-                     and has been ignored.", category)
-        }).collect();
 
     // Upload the crate to S3
     let mut handle = req.app().handle();
@@ -808,7 +803,13 @@ pub fn new(req: &mut Request) -> CargoResult<Response> {
     bomb.path = None;
 
     #[derive(RustcEncodable)]
-    struct R { krate: EncodableCrate, warnings: Vec<String> }
+    struct Warnings { invalid_categories: Vec<String> }
+    let warnings = Warnings {
+        invalid_categories: ignored_invalid_categories,
+    };
+
+    #[derive(RustcEncodable)]
+    struct R { krate: EncodableCrate, warnings: Warnings }
     Ok(req.json(&R { krate: krate.minimal_encodable(), warnings: warnings }))
 }
 
diff --git a/src/tests/krate.rs b/src/tests/krate.rs
index 7b05ff4d92..2f9f75653b 100644
--- a/src/tests/krate.rs
+++ b/src/tests/krate.rs
@@ -26,7 +26,9 @@ struct CrateMeta { total: i32 }
 #[derive(RustcDecodable)]
 struct GitCrate { name: String, vers: String, deps: Vec<String>, cksum: String }
 #[derive(RustcDecodable)]
-struct GoodCrate { krate: EncodableCrate, warnings: Vec<String> }
+struct Warnings { invalid_categories: Vec<String> }
+#[derive(RustcDecodable)]
+struct GoodCrate { krate: EncodableCrate, warnings: Warnings }
 #[derive(RustcDecodable)]
 struct CrateResponse { krate: EncodableCrate, versions: Vec<EncodableVersion>, keywords: Vec<EncodableKeyword> }
 #[derive(RustcDecodable)]
@@ -896,7 +898,7 @@ fn good_categories() {
     let json: GoodCrate = ::json(&mut response);
     assert_eq!(json.krate.name, "foo");
     assert_eq!(json.krate.max_version, "1.0.0");
-    assert_eq!(json.warnings.len(), 0);
+    assert_eq!(json.warnings.invalid_categories.len(), 0);
 }
 
 #[test]
@@ -910,8 +912,7 @@ fn ignored_categories() {
     let json: GoodCrate = ::json(&mut response);
     assert_eq!(json.krate.name, "foo");
     assert_eq!(json.krate.max_version, "1.0.0");
-    assert_eq!(json.warnings, vec!["\'bar\' is not a recognized category name \
-                                    and has been ignored.".to_string()]);
+    assert_eq!(json.warnings.invalid_categories, vec!["bar".to_string()]);
 }
 
 #[test]