diff --git a/src/fn_call.rs b/src/fn_call.rs
index 7799a8af27..ec3f1d9840 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -615,7 +615,7 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
                 this.write_scalar(handle, dest)?;
             }
             "WriteFile" => {
-                let handle = this.read_scalar(args[0])?.to_i32()?;
+                let handle = this.read_scalar(args[0])?.to_isize()?;
                 let buf = this.read_scalar(args[1])?.not_undef()?;
                 let n = this.read_scalar(args[2])?.to_usize(&*this.tcx)?;
                 let written_place = this.deref_operand(args[3])?;