diff --git a/src/fn_call.rs b/src/fn_call.rs
index f8ab005506..4d39625a7e 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -596,12 +596,12 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
                 this.write_scalar(Scalar::from_uint(key, dest.layout.size), dest)?;
             }
             "TlsGetValue" => {
-                let key = this.read_scalar(args[0])?.to_bits(args[0].layout.size)?;
+                let key = this.read_scalar(args[0])?.to_u32()? as u128;
                 let ptr = this.machine.tls.load_tls(key)?;
                 this.write_scalar(ptr, dest)?;
             }
             "TlsSetValue" => {
-                let key = this.read_scalar(args[0])?.to_bits(args[0].layout.size)?;
+                let key = this.read_scalar(args[0])?.to_u32()? as u128;
                 let new_ptr = this.read_scalar(args[1])?.not_undef()?;
                 this.machine.tls.store_tls(key, new_ptr)?;
 
@@ -615,22 +615,22 @@ 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(this)?;
                 let buf = this.read_scalar(args[1])?.not_undef()?;
-                let n = this.read_scalar(args[2])?.to_usize(&*this.tcx)?;
+                let n = this.read_scalar(args[2])?.to_u32()?;
                 let written_place = this.deref_operand(args[3])?;
                 this.write_null(written_place.into())?; // spec says we always write 0 first
                 let written = if handle == -11 || handle == -12 {
                     // stdout/stderr
                     use std::io::{self, Write};
 
-                    let buf_cont = this.memory().read_bytes(buf, Size::from_bytes(n))?;
+                    let buf_cont = this.memory().read_bytes(buf, Size::from_bytes(u64::from(n)))?;
                     let res = if handle == -11 {
                         io::stdout().write(buf_cont)
                     } else {
                         io::stderr().write(buf_cont)
                     };
-                    res.ok().map(|n| n as u64)
+                    res.ok().map(|n| n as u32)
                 } else {
                     eprintln!("Miri: Ignored output to handle {}", handle);
                     Some(n) // pretend it all went well