diff --git a/blake2b.js b/blake2b.js
index e94c50e..0c42be7 100644
--- a/blake2b.js
+++ b/blake2b.js
@@ -238,6 +238,9 @@ function blake2bInit (outlen, key, salt, personal) {
   if (outlen === 0 || outlen > 64) {
     throw new Error('Illegal output length, expected 0 < length <= 64')
   }
+  if (key && !(key instanceof Uint8Array)) {
+    throw new Error(`Illegal key, expected Uint8Array with 0 < length <= 64, got ${typeof key}`)
+  }
   if (key && key.length > 64) {
     throw new Error('Illegal key, expected Uint8Array with 0 < length <= 64')
   }
diff --git a/blake2s.js b/blake2s.js
index 517b136..39dec9b 100644
--- a/blake2s.js
+++ b/blake2s.js
@@ -258,6 +258,9 @@ function blake2sInit (outlen, key) {
   if (!(outlen > 0 && outlen <= 32)) {
     throw new Error('Incorrect output length, should be in [1, 32]')
   }
+  if (key && !(key instanceof Uint8Array)) {
+    throw new Error(`Illegal key, expected Uint8Array with 0 < length <= 32, got ${typeof key}`)
+  }
   const keylen = key ? key.length : 0
   if (key && !(keylen > 0 && keylen <= 32)) {
     throw new Error('Incorrect key length, should be in [1, 32]')
diff --git a/test_blake2b.js b/test_blake2b.js
index c3069d4..e7da735 100644
--- a/test_blake2b.js
+++ b/test_blake2b.js
@@ -35,6 +35,23 @@ test('Input types', function (t) {
   t.end()
 })
 
+test('Does not accept non-Uint8 data as "key" parameter', function (t) {
+  t.throws(() => {
+    blake2bHex('The quick brown fox jumps over the lazy dog', 'aStringKey')
+  },
+  /Illegal key, expected Uint8Array with 0 < length <= 64, got string/
+  )
+
+  t.throws(() => {
+    const nonUint8Array = ['nonSensicalValue']
+    blake2bHex('The quick brown fox jumps over the lazy dog', nonUint8Array)
+  },
+  'Illegal key, expected Uint8Array with 0 < length <= 64, got array'
+  )
+
+  t.end()
+})
+
 test('BLAKE2b generated test vectors', function (t) {
   const contents = fs.readFileSync('generated_test_vectors.txt', 'utf8')
   contents.split('\n').forEach(function (line) {
diff --git a/test_blake2s.js b/test_blake2s.js
index c4ffa86..5e9d15a 100644
--- a/test_blake2s.js
+++ b/test_blake2s.js
@@ -70,6 +70,23 @@ function generateInput (len, seed) {
   return out
 }
 
+test('Does not accept non-Uint8 data as "key" parameter', function (t) {
+  t.throws(() => {
+    blake2sHex('The quick brown fox jumps over the lazy dog', 'aStringKey')
+  },
+  /Illegal key, expected Uint8Array with 0 < length <= 32, got string/
+  )
+
+  t.throws(() => {
+    const nonUint8Array = ['nonSensicalValue']
+    blake2sHex('The quick brown fox jumps over the lazy dog', nonUint8Array)
+  },
+  'Illegal key, expected Uint8Array with 0 < length <= 32, got array'
+  )
+
+  t.end()
+})
+
 test('BLAKE2s performance', function (t) {
   const N = 1 << 22 // number of bytes to hash
   const RUNS = 3 // how often to repeat, to allow JIT to finish