diff --git a/Cargo.toml b/Cargo.toml
index 2b4578a..5f1616f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,7 +3,7 @@ authors = ["Kristoffer Ström <kristoffer@rymdkoloni.se>",
            "David Stainton <dawuud@riseup.net>"]
 edition = "2018"
 name = "appendix"
-version = "0.2.0"
+version = "0.2.1"
 description = "Append-only, on-disk key-value index"
 documentation = "https://docs.rs/appendix/"
 keywords = ["index", "datastructure"]
diff --git a/src/lib.rs b/src/lib.rs
index db44f31..b8149c0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -58,8 +58,18 @@ pub struct Index<K, V> {
     _marker: PhantomData<(K, V)>,
 }
 
-unsafe impl<K, V> Send for Index<K, V> {}
-unsafe impl<K, V> Sync for Index<K, V> {}
+unsafe impl<K, V> Send for Index<K, V>
+where
+    K: Send,
+    V: Send,
+{
+}
+unsafe impl<K, V> Sync for Index<K, V>
+where
+    K: Sync,
+    V: Sync,
+{
+}
 
 #[derive(Debug)]
 #[repr(C)]