diff --git a/src/distro_info.rs b/src/distro_info.rs
index 94b88e3..b0a6b20 100644
--- a/src/distro_info.rs
+++ b/src/distro_info.rs
@@ -220,18 +220,24 @@ pub async fn get_ordered_series_name(dist: &str) -> Result<Vec<String>, Box<dyn
 /// the development series of `dist`, i.e. the first entry of
 /// [`get_ordered_series_name`] (which is documented "development series
 /// first"). UNRELEASED work conventionally targets the next release, not
-/// the last stable one. Any other `series` is returned unchanged. Errors
+/// the last stable one. `dist` is matched case-insensitively, so vendor
+/// names with original casing (dpkg's `Vendor:` field is e.g. "Ubuntu")
+/// are accepted as-is. Any other `series` is returned unchanged. Errors
 /// when `dist` is unknown or has no series list.
 pub async fn effective_series(series: &str, dist: &str) -> Result<String, Box<dyn Error>> {
     if !is_unreleased(series) {
         return Ok(series.to_string());
     }
 
-    get_ordered_series_name(dist)
+    // The series data keys are lowercase, unlike the vendor names that
+    // callers typically resolve from dpkg
+    let dist = dist.to_lowercase();
+
+    get_ordered_series_name(&dist)
         .await?
         .into_iter()
         .next()
-        .ok_or_else(|| format!("Distribution '{}' has no series to target", dist).into())
+        .ok_or_else(|| format!("Distribution '{dist}' has no series to target").into())
 }
 
 /// Get the latest released series for a dist (excluding future releases and special cases like sid)
@@ -587,6 +593,21 @@ mod tests {
         }
     }
 
+    #[tokio::test]
+    async fn test_effective_series_unreleased_dist_case_insensitive() {
+        // Distro data keys are lowercase but dpkg vendors keep original
+        // casing ("Ubuntu"): the UNRELEASED lookup must resolve both
+        let expected = effective_series(UNRELEASED, "ubuntu").await.unwrap();
+        assert_eq!(
+            effective_series(UNRELEASED, "Ubuntu").await.unwrap(),
+            expected
+        );
+        assert_eq!(
+            effective_series(UNRELEASED, "UBUNTU").await.unwrap(),
+            expected
+        );
+    }
+
     #[tokio::test]
     async fn test_effective_series_unknown_dist() {
         // UNRELEASED on an unknown distribution cannot be resolved
diff --git a/src/main.rs b/src/main.rs
index 6c245b5..5bee718 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -206,7 +206,9 @@ fn main() {
                         // its own distribution.
                         match rt.block_on(async {
                             if pkh::distro_info::is_unreleased(&current_series) {
-                                let dist = pkh::build::env::current_vendor();
+                                // Vendors keep original casing ("Ubuntu"),
+                                // while the series data keys are lowercase
+                                let dist = pkh::build::env::current_vendor().to_lowercase();
                                 let mut series_list =
                                     vec![pkh::distro_info::UNRELEASED.to_string()];
                                 series_list.extend(