diff --git a/lib/rbnacl/signatures/ed25519/signing_key.rb b/lib/rbnacl/signatures/ed25519/signing_key.rb
index bc2275d..6693ca0 100644
--- a/lib/rbnacl/signatures/ed25519/signing_key.rb
+++ b/lib/rbnacl/signatures/ed25519/signing_key.rb
@@ -77,7 +77,11 @@ def initialize(seed)
         #
         # @return [String] Signature as bytes
         def sign(message)
-          sign_attached(message)[0, signature_bytes]
+          # Force message to binary encoding to ensure [0, signature_bytes] slices by bytes not characters
+          # This prevents issues with UTF-8 encoded strings where multi-byte characters would cause
+          # incorrect signature length (e.g., 90 bytes instead of 64)
+          message_binary = message.b
+          sign_attached(message_binary)[0, signature_bytes]
         end
 
         # Sign a message using this key, attaching the signature to the message
diff --git a/lib/rbnacl/signatures/ed25519/verify_key.rb b/lib/rbnacl/signatures/ed25519/verify_key.rb
index 8282c1a..f7a5055 100644
--- a/lib/rbnacl/signatures/ed25519/verify_key.rb
+++ b/lib/rbnacl/signatures/ed25519/verify_key.rb
@@ -51,7 +51,10 @@ def initialize(key)
         def verify(signature, message)
           signature = signature.to_str
           Util.check_length(signature, signature_bytes, "signature")
-          verify_attached(signature + message)
+          # Force message to binary encoding to prevent encoding compatibility errors
+          # when concatenating with signature (which is always ASCII-8BIT)
+          message_binary = message.b
+          verify_attached(signature + message_binary)
         end
 
         # Verify a signature for a given signed message
diff --git a/spec/rbnacl/signatures/ed25519/signing_key_spec.rb b/spec/rbnacl/signatures/ed25519/signing_key_spec.rb
index ab54e7e..cc6d330 100644
--- a/spec/rbnacl/signatures/ed25519/signing_key_spec.rb
+++ b/spec/rbnacl/signatures/ed25519/signing_key_spec.rb
@@ -18,6 +18,13 @@
     expect(subject.sign(message)).to eq signature
   end
 
+  it "signs UTF-8 encoded messages" do
+    utf8_message = "Björn Müller from München".dup.force_encoding("UTF-8")
+    signature = subject.sign(utf8_message)
+    expect(signature.bytesize).to eq(64)
+    expect(subject.verify_key.verify(signature, utf8_message)).to be true
+  end
+
   it "signs messages, full version" do
     expect(subject.sign_attached(message)[0, RbNaCl::SigningKey.signature_bytes]).to eq signature
     expect(subject.sign_attached(message)[RbNaCl::SigningKey.signature_bytes, message.length]).to eq message