diff --git a/src/libcore/vec.rs b/src/libcore/vec.rs
index 09292fc11612..b1bd92c5b1e7 100644
--- a/src/libcore/vec.rs
+++ b/src/libcore/vec.rs
@@ -773,11 +773,19 @@ fn iter_between<T>(v: [const T], start: uint, end: uint, f: fn(T)) {
     }
 }
 
-#[doc = "Iterates over two vectors in parallel"]
+#[doc = "
+Iterates over two vectors simultaneously
+
+# Failure
+
+Both vectors must have the same length
+"]
 #[inline]
-fn iter2<U, T>(v: [ U], v2: [const T], f: fn(U, T)) {
-    let mut i = 0;
-    for elt in v { f(elt, v2[i]); i += 1; }
+fn iter2<U, T>(v1: [const U], v2: [const T], f: fn(U, T)) {
+    assert len(v1) == len(v2);
+    uint::range(0u, len(v1)) {|i|
+        f(v1[i], v2[i])
+    }
 }
 
 #[doc = "