diff --git a/lib/Base/Int.fram b/lib/Base/Int.fram
index 27afad2..a9a176c 100644
--- a/lib/Base/Int.fram
+++ b/lib/Base/Int.fram
@@ -21,11 +21,11 @@ pub method sub = (extern dbl_subInt : Int -> Int -> Int) self
 pub method mul = (extern dbl_mulInt : Int -> Int -> Int) self
 
 pub method div (n : Int) =
-  assert {msg="Division by zero"} (n.neq 0);
+  assert {msg="Division by zero"} (n != 0);
   (extern dbl_divInt : Int -> Int -> Int) self n
 
 pub method mod (n : Int) =
-  assert {msg="Division by zero"} (n.neq 0);
+  assert {msg="Division by zero"} (n != 0);
   (extern dbl_modInt : Int -> Int -> Int) self n
 
 pub method neg {self : Int} = 0 - self
diff --git a/lib/Base/Int64.fram b/lib/Base/Int64.fram
index 621ef03..b83fd95 100644
--- a/lib/Base/Int64.fram
+++ b/lib/Base/Int64.fram
@@ -48,14 +48,12 @@ pub method toString {self : Int64} =
   (extern dbl_int64ToString : Int64 -> String) self
 pub method toIntErr { ~onError, self : Int64 } =
   if self <= 0x3fffffffffffffffL && self >= 0xc000000000000000L then
-    (extern dbl_intToInt64 : Int64 -> Int) self
+    (extern dbl_int64ToInt : Int64 -> Int) self
   else ~onError ()
-pub method toIntOpt {self : Int64} =
+pub method toInt {self : Int64} =
   if self <= 0x3fffffffffffffffL && self >= 0x7fffffffffffffffL then
     Some ((extern dbl_int64ToInt : Int64 -> Int) self)
   else None
-pub method toInt {self : Int64} =
-  self.toIntErr { ~onError = fn () => runtimeError "Conversion overflow" }
 
 pub method abs {self : Int64} =
   if self < 0L then self.neg else self
diff --git a/lib/Base/Option.fram b/lib/Base/Option.fram
new file mode 100644
index 0000000..abd4de3
--- /dev/null
+++ b/lib/Base/Option.fram
@@ -0,0 +1,25 @@
+{# This file is part of DBL, released under MIT license.
+ # See LICENSE for details.
+ #}
+
+import open /Base/Types
+import open /Base/Assert
+
+{## For `Some x` returns `x`, otherwise the provided argument is returned.
+ #}
+pub method unwrapOr default =
+  match self with
+  | None   => default
+  | Some x => x
+  end
+
+{## This method should only be called on `Some x` values, in which case it
+  returns `x`. When applied to `None` the entire program crashes irrecoverably
+  with a runtime error. The resulting error message can be optionally specified
+  with `?msg`.
+ #}
+pub method unwrap {?msg} =
+  match self with
+  | None   => runtimeError (msg.unwrapOr "Called `unwrap` on `None`")
+  | Some x => x
+  end
diff --git a/lib/Prelude.fram b/lib/Prelude.fram
index eb17828..77618c8 100644
--- a/lib/Prelude.fram
+++ b/lib/Prelude.fram
@@ -6,6 +6,7 @@ import /Base/Types
 import /Base/Operators
 import /Base/Assert
 import /Base/Bool
+import /Base/Option
 import /Base/Int
 import /Base/Int64
 import /Base/Char
diff --git a/src/Eval/External.ml b/src/Eval/External.ml
index 4614441..bf3fbe2 100644
--- a/src/Eval/External.ml
+++ b/src/Eval/External.ml
@@ -50,7 +50,7 @@ let int_cmpop op = int_fun2 (fun x y -> of_bool (op x y))
 let int64_fun f = VFn (fun v cont ->
   match v with
   | VNum64 n -> cont (f n)
-  | _ -> failwith "Not a 64-bit integer")
+  | _ -> runtime_error "Not a 64-bit integer")
 
 let int64_fun2 f = int64_fun (fun x -> int64_fun (f x))