diff --git a/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/HMAC.kt b/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/HMAC.kt
index 7b42f868..85e14a26 100644
--- a/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/HMAC.kt
+++ b/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/HMAC.kt
@@ -3,19 +3,17 @@ package org.jetbrains.kotlinx.jupyter.protocol
 import javax.crypto.Mac
 import javax.crypto.spec.SecretKeySpec
 
-class HMAC(algorithm: String, key: String?) {
-    private val mac = if (key?.isNotBlank() == true) Mac.getInstance(algorithm) else null
-
-    init {
-        mac?.init(SecretKeySpec(key!!.toByteArray(), algorithm))
-    }
+class HMAC(algorithm: String, key: String) {
+    private val mac: Mac =
+        Mac.getInstance(algorithm).apply {
+            init(SecretKeySpec(key.toByteArray(), algorithm))
+        }
 
     @Synchronized
-    operator fun invoke(data: Iterable<ByteArray>): String? =
-        mac?.let { mac ->
-            data.forEach { mac.update(it) }
-            mac.doFinal().toHexString()
-        }
+    operator fun invoke(data: Iterable<ByteArray>): String {
+        data.forEach { mac.update(it) }
+        return mac.doFinal().toHexString()
+    }
 
-    operator fun invoke(vararg data: ByteArray): String? = invoke(data.asIterable())
+    operator fun invoke(vararg data: ByteArray): String = invoke(data.asIterable())
 }
diff --git a/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/SocketWrapper.kt b/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/SocketWrapper.kt
index b581892d..654f0601 100644
--- a/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/SocketWrapper.kt
+++ b/jupyter-lib/shared-compiler/src/main/kotlin/org/jetbrains/kotlinx/jupyter/protocol/SocketWrapper.kt
@@ -89,7 +89,7 @@ class SocketWrapper(
                     prop ->
                 prop.get(msg)?.let { MessageFormat.encodeToString(it) }?.toByteArray() ?: emptyJsonObjectStringBytes
             }
-        sendMore(hmac(signableMsg) ?: "")
+        sendMore(hmac(signableMsg))
         for (i in 0 until (signableMsg.size - 1)) {
             sendMore(signableMsg[i])
         }
@@ -121,7 +121,7 @@ class SocketWrapper(
         val content = recv()
         val calculatedSig = hmac(header, parentHeader, metadata, content)
 
-        if (calculatedSig != null && sig != calculatedSig) {
+        if (sig != calculatedSig) {
             throw SignatureException("Invalid signature: expected $calculatedSig, received $sig - $ids")
         }
 
diff --git a/src/test/kotlin/org/jetbrains/kotlinx/jupyter/test/protocol/KernelServerTestsBase.kt b/src/test/kotlin/org/jetbrains/kotlinx/jupyter/test/protocol/KernelServerTestsBase.kt
index e774173e..bc88e8c1 100644
--- a/src/test/kotlin/org/jetbrains/kotlinx/jupyter/test/protocol/KernelServerTestsBase.kt
+++ b/src/test/kotlin/org/jetbrains/kotlinx/jupyter/test/protocol/KernelServerTestsBase.kt
@@ -41,7 +41,7 @@ abstract class KernelServerTestsBase(protected val runServerInSeparateProcess: B
     protected val kernelConfig =
         createKotlinKernelConfig(
             ports = createRandomKernelPorts(),
-            signatureKey = "",
+            signatureKey = "abc",
             scriptClasspath = classpath,
             homeDir = File(""),
         )