diff --git a/src/mapped.rs b/src/mapped.rs
index 5f35469..83491c1 100644
--- a/src/mapped.rs
+++ b/src/mapped.rs
@@ -7,6 +7,7 @@ use crate::{FilePointer, FileRange, RawFilePointer};
 pub trait ReaderTrait {
     fn read_raw(&self, range: FileRange) -> &[u8];
     fn read<T: FromBytes>(&self, ptr: FilePointer<T>) -> &T {
+        debug_assert!(!ptr.is_null(), "null pointer dereference");
         let bytes = self.read_raw(ptr.range());
         Ref::<_, T>::new(bytes)
             .unwrap_or_else(|| {