diff --git a/fitsio/src/headers.rs b/fitsio/src/headers.rs
index 50b72d0d..60fd480b 100644
--- a/fitsio/src/headers.rs
+++ b/fitsio/src/headers.rs
@@ -75,7 +75,7 @@ impl ReadsKey for bool {
     where
         Self: Sized,
     {
-        let int_value = i32::read_key(f, name)?;
+        let int_value = i64::read_key(f, name)?;
         Ok(int_value > 0)
     }
 }