diff --git a/src/shims/foreign_items/posix.rs b/src/shims/foreign_items/posix.rs
index 39b00feec194..09191011a4a8 100644
--- a/src/shims/foreign_items/posix.rs
+++ b/src/shims/foreign_items/posix.rs
@@ -170,7 +170,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             // Dynamic symbol loading
             "dlsym" => {
                 let &[handle, symbol] = check_arg_count(args)?;
-                this.read_scalar(handle)?.not_undef()?;
+                this.read_scalar(handle)?.to_machine_usize(this)?;
                 let symbol = this.read_scalar(symbol)?.not_undef()?;
                 let symbol_name = this.memory.read_c_str(symbol)?;
                 if let Some(dlsym) = Dlsym::from_str(symbol_name, &this.tcx.sess.target.target.target_os)? {
@@ -369,9 +369,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             }
             "pthread_atfork" => {
                 let &[prepare, parent, child] = check_arg_count(args)?;
-                this.read_scalar(prepare)?.not_undef()?;
-                this.read_scalar(parent)?.not_undef()?;
-                this.read_scalar(child)?.not_undef()?;
+                this.force_bits(this.read_scalar(prepare)?.not_undef()?, this.memory.pointer_size())?;
+                this.force_bits(this.read_scalar(parent)?.not_undef()?, this.memory.pointer_size())?;
+                this.force_bits(this.read_scalar(child)?.not_undef()?, this.memory.pointer_size())?;
                 // We do not support forking, so there is nothing to do here.
                 this.write_null(dest)?;
             }
diff --git a/src/shims/foreign_items/posix/macos.rs b/src/shims/foreign_items/posix/macos.rs
index 1cfecbc93461..e6d39af45395 100644
--- a/src/shims/foreign_items/posix/macos.rs
+++ b/src/shims/foreign_items/posix/macos.rs
@@ -105,13 +105,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             // Querying system information
             "pthread_get_stackaddr_np" => {
                 let &[thread] = check_arg_count(args)?;
-                this.read_scalar(thread)?.not_undef()?;
+                this.read_scalar(thread)?.to_machine_usize(this)?;
                 let stack_addr = Scalar::from_uint(STACK_ADDR, this.pointer_size());
                 this.write_scalar(stack_addr, dest)?;
             }
             "pthread_get_stacksize_np" => {
                 let &[thread] = check_arg_count(args)?;
-                this.read_scalar(thread)?.not_undef()?;
+                this.read_scalar(thread)?.to_machine_usize(this)?;
                 let stack_size = Scalar::from_uint(STACK_SIZE, this.pointer_size());
                 this.write_scalar(stack_size, dest)?;
             }
diff --git a/src/shims/foreign_items/windows.rs b/src/shims/foreign_items/windows.rs
index 60448406a67d..c24824153ca2 100644
--- a/src/shims/foreign_items/windows.rs
+++ b/src/shims/foreign_items/windows.rs
@@ -210,7 +210,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             "GetProcAddress" => {
                 #[allow(non_snake_case)]
                 let &[hModule, lpProcName] = check_arg_count(args)?;
-                this.read_scalar(hModule)?.not_undef()?;
+                this.read_scalar(hModule)?.to_machine_isize(this)?;
                 let name = this.memory.read_c_str(this.read_scalar(lpProcName)?.not_undef()?)?;
                 if let Some(dlsym) = Dlsym::from_str(name, &this.tcx.sess.target.target.target_os)? {
                     let ptr = this.memory.create_fn_alloc(FnVal::Other(dlsym));