diff --git a/rslib/src/notetype/schema11.rs b/rslib/src/notetype/schema11.rs
index 5a4238eb5..79f29a1f2 100644
--- a/rslib/src/notetype/schema11.rs
+++ b/rslib/src/notetype/schema11.rs
@@ -264,7 +264,7 @@ pub struct CardTemplateSchema11 {
     pub(crate) bqfmt: String,
     #[serde(default)]
     pub(crate) bafmt: String,
-    #[serde(deserialize_with = "default_on_invalid")]
+    #[serde(deserialize_with = "default_on_invalid", default)]
     pub(crate) did: Option<DeckID>,
     #[serde(default, deserialize_with = "default_on_invalid")]
     pub(crate) bfont: String,
diff --git a/rslib/src/serde.rs b/rslib/src/serde.rs
index 4b202a17f..6f61627ee 100644
--- a/rslib/src/serde.rs
+++ b/rslib/src/serde.rs
@@ -8,6 +8,8 @@ pub(crate) use serde_aux::field_attributes::{
 };
 use serde_json::Value;
 
+/// Note: if you wish to cover the case where a field is missing, make sure you also
+/// use the `serde(default)` flag.
 pub(crate) fn default_on_invalid<'de, T, D>(deserializer: D) -> Result<T, D::Error>
 where
     T: Default + DeTrait<'de>,
@@ -63,3 +65,31 @@ impl FromI64 for TimestampSecs {
         TimestampSecs(val as i64)
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use serde::Deserialize;
+
+    #[derive(Deserialize, Debug, PartialEq)]
+    struct MaybeInvalid {
+        #[serde(deserialize_with = "default_on_invalid", default)]
+        field: Option<usize>,
+    }
+
+    #[test]
+    fn invalid_or_missing() {
+        assert_eq!(
+            serde_json::from_str::<MaybeInvalid>(r#"{"field": 5}"#).unwrap(),
+            MaybeInvalid { field: Some(5) }
+        );
+        assert_eq!(
+            serde_json::from_str::<MaybeInvalid>(r#"{"field": "5"}"#).unwrap(),
+            MaybeInvalid { field: None }
+        );
+        assert_eq!(
+            serde_json::from_str::<MaybeInvalid>(r#"{"another": 5}"#).unwrap(),
+            MaybeInvalid { field: None }
+        );
+    }
+}