diff --git a/src/fingerprint.rs b/src/fingerprint.rs
index e6dc25bf..727fc155 100644
--- a/src/fingerprint.rs
+++ b/src/fingerprint.rs
@@ -1,6 +1,6 @@
 use super::*;
 
-#[derive(Clone, Copy, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct Fingerprint(pub(crate) Hash);
 
 impl Fingerprint {
@@ -19,9 +19,22 @@ impl Display for Fingerprint {
 }
 
 impl FromStr for Fingerprint {
-  type Err = blake3::HexError;
+  type Err = HashError;
 
   fn from_str(s: &str) -> Result<Self, Self::Err> {
     Ok(Self(s.parse()?))
   }
 }
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn uppercase_is_forbidden() {
+    test::HASH
+      .to_uppercase()
+      .parse::<Fingerprint>()
+      .unwrap_err();
+  }
+}
diff --git a/src/hash.rs b/src/hash.rs
index 5205c588..11e1a16a 100644
--- a/src/hash.rs
+++ b/src/hash.rs
@@ -30,10 +30,16 @@ impl From<[u8; Hash::LEN]> for Hash {
 }
 
 impl FromStr for Hash {
-  type Err = blake3::HexError;
+  type Err = HashError;
 
   fn from_str(s: &str) -> Result<Self, Self::Err> {
-    Ok(Self(s.parse()?))
+    let hash = s.parse()?;
+
+    if !is_lowercase_hex(s) {
+      return Err(hash_error::Case { hash: s }.build());
+    }
+
+    Ok(Self(hash))
   }
 }
 
@@ -103,4 +109,9 @@ mod tests {
     );
     assert_eq!(serde_json::from_str::<Hash>(&json).unwrap(), input);
   }
+
+  #[test]
+  fn uppercase_is_forbidden() {
+    test::HASH.to_uppercase().parse::<Hash>().unwrap_err();
+  }
 }
diff --git a/src/hash_error.rs b/src/hash_error.rs
new file mode 100644
index 00000000..c3630b0c
--- /dev/null
+++ b/src/hash_error.rs
@@ -0,0 +1,10 @@
+use super::*;
+
+#[derive(Debug, Snafu)]
+#[snafu(context(suffix(false)), visibility(pub(crate)))]
+pub enum HashError {
+  #[snafu(display("hashes must be lowercase hex: `{hash}`"))]
+  Case { hash: String },
+  #[snafu(transparent)]
+  Hex { source: blake3::HexError },
+}
diff --git a/src/lib.rs b/src/lib.rs
index 21c5f0c4..74dba491 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -31,6 +31,7 @@ use {
     fingerprint_prefix::FingerprintPrefix,
     format::Format,
     functions::{current_dir, decode_path, default, is_default, is_lowercase_hex, now},
+    hash_error::HashError,
     index::Index,
     key_identifier::KeyIdentifier,
     key_name::KeyName,
@@ -142,6 +143,7 @@ mod fingerprint_prefix;
 mod format;
 mod functions;
 mod hash;
+mod hash_error;
 mod index;
 mod key_identifier;
 mod key_name;