diff --git a/src/helpers.rs b/src/helpers.rs
index c975ad144e26..ddaf8f48a5aa 100644
--- a/src/helpers.rs
+++ b/src/helpers.rs
@@ -356,19 +356,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let len = bytes.len();
         // If `size` is smaller or equal than `bytes.len()`, writing `bytes` plus the required null
         // terminator to memory using the `ptr` pointer would cause an overflow.
-        if (len as u64) < size {
-            let this = self.eval_context_mut();
-            let tcx = &{ this.tcx.tcx };
-            let buffer = this.memory.get_mut(ptr.alloc_id)?.get_bytes_mut(tcx, ptr, Size::from_bytes(len as u64 + 1))?;
-            buffer[..len].copy_from_slice(bytes);
-            // This is ok because the buffer was strictly larger than `bytes`, so after adding the
-            // null terminator, the buffer size is larger or equal to `bytes.len()`, meaning that
-            // `bytes` actually fit inside tbe buffer.
-            buffer[len] = 0;
-            Ok(())
-        } else {
-            throw_unsup_format!("OsString is larger than destination")
+        if size <= bytes.len() as u64 {
+            throw_unsup_format!("OsString of length {} is too large for destination buffer of size {}", len, size)
         }
+
+        let this = self.eval_context_mut();
+        let buffer = this.memory.get_mut(ptr.alloc_id)?.get_bytes_mut(&*this.tcx, ptr, Size::from_bytes(len as u64 + 1))?;
+        buffer[..len].copy_from_slice(bytes);
+        // This is ok because the buffer was strictly larger than `bytes`, so after adding the
+        // null terminator, the buffer size is larger or equal to `bytes.len()`, meaning that
+        // `bytes` actually fit inside tbe buffer.
+        buffer[len] = 0;
+        Ok(())
     }
 }