diff --git a/README.md b/README.md
index 800b19d..36daa2a 100644
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@ To access an existing OCI directory:
 # fn main() -> anyhow::Result<()> {
 let d = cap_std::fs::Dir::open_ambient_dir("/path/to/ocidir", cap_std::ambient_authority())?;
 let d = ocidir::OciDir::open(d)?;
-println!("{:?}", d.read_index()?.ok_or_else(|| anyhow!("missing Image Index"))?);
+println!("{:?}", d.read_index()?);
 # Ok(())
 # }
 ```
diff --git a/src/lib.rs b/src/lib.rs
index 25e84f7..1f160be 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -54,6 +54,9 @@ pub enum Error {
     #[error("Expected digest algorithm sha256 but found {found}")]
     /// Returned when a digest algorithm is not supported
     UnsupportedDigestAlgorithm { found: Box<str> },
+    #[error("Cannot find the Image Index (index.json)")]
+    /// Returned when the OCI Image Index (index.json) is missing
+    MissingImageIndex,
     #[error("error")]
     /// An unknown other error
     Other(Box<str>),
@@ -384,9 +387,7 @@ impl OciDir {
 
     /// Returns `true` if the manifest is already present.
     pub fn has_manifest(&self, desc: &oci_spec::image::Descriptor) -> Result<bool> {
-        let Some(index) = self.read_index()? else {
-            return Ok(false);
-        };
+        let index = self.read_index()?;
         Ok(index
             .manifests()
             .iter()
@@ -414,11 +415,11 @@ impl OciDir {
     }
 
     /// Read the image index.
-    pub fn read_index(&self) -> Result<Option<ImageIndex>> {
+    pub fn read_index(&self) -> Result<ImageIndex> {
         let r = if let Some(index) = self.dir.open_optional("index.json")?.map(BufReader::new) {
-            Some(oci_image::ImageIndex::from_reader(index)?)
+            oci_image::ImageIndex::from_reader(index)?
         } else {
-            None
+            return Err(Error::MissingImageIndex);
         };
         Ok(r)
     }
@@ -442,21 +443,23 @@ impl OciDir {
             manifest.set_annotations(Some(annotations));
         }
 
-        let index = self.read_index()?;
-        let index = if let Some(mut index) = index {
-            let mut manifests = index.manifests().clone();
-            if let Some(tag) = tag {
-                manifests.retain(|d| !Self::descriptor_is_tagged(d, tag));
+        let index = match self.read_index() {
+            Ok(mut index) => {
+                let mut manifests = index.manifests().clone();
+                if let Some(tag) = tag {
+                    manifests.retain(|d| !Self::descriptor_is_tagged(d, tag));
+                }
+                manifests.push(manifest.clone());
+                index.set_manifests(manifests);
+                index
             }
-            manifests.push(manifest.clone());
-            index.set_manifests(manifests);
-            index
-        } else {
-            oci_image::ImageIndexBuilder::default()
+            Err(Error::MissingImageIndex) => oci_image::ImageIndexBuilder::default()
                 .schema_version(oci_image::SCHEMA_VERSION)
                 .manifests(vec![manifest.clone()])
-                .build()
-                .unwrap()
+                .build()?,
+            Err(e) => {
+                return Err(e);
+            }
         };
 
         self.dir
@@ -519,14 +522,21 @@ impl OciDir {
 
     /// Find the manifest with the provided tag
     pub fn find_manifest_with_tag(&self, tag: &str) -> Result<Option<oci_image::ImageManifest>> {
-        let f = self.dir.open("index.json")?;
-        let idx: oci_image::ImageIndex = serde_json::from_reader(BufReader::new(f))?;
-        for img in idx.manifests() {
-            if Self::descriptor_is_tagged(img, tag) {
-                return self.read_json_blob(img).map(Some);
-            }
-        }
-        Ok(None)
+        let desc = self.find_manifest_descriptor_with_tag(tag)?;
+        desc.map(|img| self.read_json_blob(&img)).transpose()
+    }
+
+    /// Find the manifest descriptor with the provided tag
+    pub fn find_manifest_descriptor_with_tag(
+        &self,
+        tag: &str,
+    ) -> Result<Option<oci_image::Descriptor>> {
+        let idx = self.read_index()?;
+        Ok(idx
+            .manifests()
+            .iter()
+            .find(|desc| Self::descriptor_is_tagged(desc, tag))
+            .cloned())
     }
 
     /// Verify a single manifest and all of its referenced objects.
@@ -567,9 +577,7 @@ impl OciDir {
     /// Verify consistency of the index, its manifests, the config and blobs (all the latter)
     /// by verifying their descriptor.
     pub fn fsck(&self) -> Result<u64> {
-        let Some(index) = self.read_index()? else {
-            return Ok(0);
-        };
+        let index = self.read_index()?;
         let mut validated_blobs = HashSet::new();
         for manifest_descriptor in index.manifests() {
             let expected_sha256 = sha256_of_descriptor(manifest_descriptor)?;
@@ -813,7 +821,7 @@ mod tests {
             "349438e5faf763e8875b43de4d7101540ef4d865190336c2cc549a11f33f8d7c"
         );
         // Nothing referencing this blob yet
-        assert_eq!(w.fsck().unwrap(), 0);
+        assert!(matches!(w.fsck().unwrap_err(), Error::MissingImageIndex));
         assert!(w.has_blob(&root_layer_desc).unwrap());
 
         // Check that we don't find nonexistent blobs
@@ -834,7 +842,7 @@ mod tests {
         let config = w.write_config(config)?;
         manifest.set_config(config);
         w.replace_with_single_manifest(manifest.clone(), oci_image::Platform::default())?;
-        assert_eq!(w.read_index().unwrap().unwrap().manifests().len(), 1);
+        assert_eq!(w.read_index().unwrap().manifests().len(), 1);
         assert_eq!(w.fsck().unwrap(), 3);
         // Also verify that corrupting a blob is found
         {
@@ -851,7 +859,7 @@ mod tests {
             assert_eq!(w.fsck().unwrap(), 3);
         }
 
-        let idx = w.read_index()?.unwrap();
+        let idx = w.read_index()?;
         let manifest_desc = idx.manifests().first().unwrap();
         let read_manifest = w.read_json_blob(manifest_desc).unwrap();
         assert_eq!(&read_manifest, &manifest);
@@ -860,7 +868,7 @@ mod tests {
             w.insert_manifest(manifest, Some("latest"), oci_image::Platform::default())?;
         assert!(w.has_manifest(&desc).unwrap());
         // There's more than one now
-        assert_eq!(w.read_index().unwrap().unwrap().manifests().len(), 2);
+        assert_eq!(w.read_index().unwrap().manifests().len(), 2);
 
         assert!(w.find_manifest_with_tag("noent").unwrap().is_none());
         let found_via_tag = w.find_manifest_with_tag("latest").unwrap().unwrap();
@@ -880,7 +888,7 @@ mod tests {
             Some("latest"),
             oci_image::Platform::default(),
         )?;
-        assert_eq!(w.read_index().unwrap().unwrap().manifests().len(), 2);
+        assert_eq!(w.read_index().unwrap().manifests().len(), 2);
         assert_eq!(w.fsck().unwrap(), 6);
         Ok(())
     }