diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index 12d38de6a..be52383ef 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.6.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.7-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
diff --git a/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/AsmBuilder.kt b/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/AsmBuilder.kt
index 06f02a94d..a1e482103 100644
--- a/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/AsmBuilder.kt
+++ b/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/AsmBuilder.kt
@@ -25,7 +25,7 @@ internal class AsmBuilder<T> internal constructor(
     private val classOfT: Class<*>,
     private val algebra: Algebra<T>,
     private val className: String,
-    private val invokeLabel0Visitor: AsmBuilder<T>.() -> Unit
+    private val invokeLabel0Visitor: AsmBuilder<T>.() -> Unit,
 ) {
     /**
      * Internal classloader of [AsmBuilder] with alias to define class from byte array.
@@ -379,22 +379,14 @@ internal class AsmBuilder<T> internal constructor(
      * Loads a variable [name] from arguments [Map] parameter of [Expression.invoke]. The [defaultValue] may be
      * provided.
      */
-    internal fun loadVariable(name: String, defaultValue: T? = null): Unit = invokeMethodVisitor.run {
+    internal fun loadVariable(name: String): Unit = invokeMethodVisitor.run {
         load(invokeArgumentsVar, MAP_TYPE)
         aconst(name)
 
-        if (defaultValue != null)
-            loadTConstant(defaultValue)
-
         invokestatic(
             MAP_INTRINSICS_TYPE.internalName,
             "getOrFail",
-
-            Type.getMethodDescriptor(
-                OBJECT_TYPE,
-                MAP_TYPE,
-                OBJECT_TYPE,
-                *OBJECT_TYPE.wrapToArrayIf { defaultValue != null }),
+            Type.getMethodDescriptor(OBJECT_TYPE, MAP_TYPE, STRING_TYPE),
             false
         )
 
@@ -429,7 +421,7 @@ internal class AsmBuilder<T> internal constructor(
         method: String,
         descriptor: String,
         expectedArity: Int,
-        opcode: Int = INVOKEINTERFACE
+        opcode: Int = INVOKEINTERFACE,
     ) {
         run loop@{
             repeat(expectedArity) {
diff --git a/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/mapIntrinsics.kt b/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/mapIntrinsics.kt
index 09e9a71b0..588b9611a 100644
--- a/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/mapIntrinsics.kt
+++ b/kmath-ast/src/jvmMain/kotlin/kscience/kmath/asm/internal/mapIntrinsics.kt
@@ -3,12 +3,11 @@
 package kscience.kmath.asm.internal
 
 import kscience.kmath.expressions.StringSymbol
+import kscience.kmath.expressions.Symbol
 
 /**
- * Gets value with given [key] or throws [IllegalStateException] whenever it is not present.
+ * Gets value with given [key] or throws [NoSuchElementException] whenever it is not present.
  *
  * @author Iaroslav Postovalov
  */
-@JvmOverloads
-internal fun <K, V> Map<K, V>.getOrFail(key: K, default: V? = null): V =
-    this[StringSymbol(key.toString())] ?: default ?: error("Parameter not found: $key")
+internal fun <V> Map<Symbol, V>.getOrFail(key: String): V = getValue(StringSymbol(key))
diff --git a/kmath-core/src/commonMain/kotlin/kscience/kmath/expressions/SimpleAutoDiff.kt b/kmath-core/src/commonMain/kotlin/kscience/kmath/expressions/SimpleAutoDiff.kt
index a718154d3..af7c8fbf2 100644
--- a/kmath-core/src/commonMain/kotlin/kscience/kmath/expressions/SimpleAutoDiff.kt
+++ b/kmath-core/src/commonMain/kotlin/kscience/kmath/expressions/SimpleAutoDiff.kt
@@ -146,7 +146,9 @@ private class AutoDiffContext<T : Any, F : Field<T>>(
         override val identity: String,
         value: T,
         var d: T,
-    ) : AutoDiffValue<T>(value), Symbol
+    ) : AutoDiffValue<T>(value), Symbol{
+        override fun toString(): String = identity
+    }
 
     private val bindings: Map<String, AutoDiffVariableWithDeriv<T>> = bindings.entries.associate {
         it.key.identity to AutoDiffVariableWithDeriv(it.key.identity, it.value, context.zero)