diff --git a/README.md b/README.md
index 68819a4..244d7d0 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@ repositories {
 }
 
 dependencies {
-    compile 'com.github.h0tk3y.betterParse:better-parse:0.3.0'
+    compile 'com.github.h0tk3y.betterParse:better-parse:0.3.1'
 }
 ```
 
diff --git a/build.gradle b/build.gradle
index bfaff09..6b2cdc6 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,5 +1,5 @@
 group 'com.github.h0tk3y.betterParse'
-version '0.3.0'
+version '0.3.1'
 
 buildscript {
     ext.kotlin_version = '1.1.51'
diff --git a/src/main/kotlin/com/github/h0tk3y/betterParse/st/LiftToSyntaxTree.kt b/src/main/kotlin/com/github/h0tk3y/betterParse/st/LiftToSyntaxTree.kt
index 821f309..fd2041d 100644
--- a/src/main/kotlin/com/github/h0tk3y/betterParse/st/LiftToSyntaxTree.kt
+++ b/src/main/kotlin/com/github/h0tk3y/betterParse/st/LiftToSyntaxTree.kt
@@ -47,7 +47,7 @@ fun <T> Parser<T>.liftToSyntaxTreeParser(
 /** Converts a [Grammar] so that its [Grammar.rootParser] parses a [SyntaxTree]. See: [liftToSyntaxTreeParser]. */
 fun <T> Grammar<T>.liftToSyntaxTreeGrammar(
     liftOptions: LiftToSyntaxTreeOptions = LiftToSyntaxTreeOptions(),
-    structureParsers: Set<Parser<*>> = declaredParsers,
+    structureParsers: Set<Parser<*>>? = declaredParsers,
     transformer: LiftToSyntaxTreeTransformer? = null
 ) = object : Grammar<SyntaxTree<T>>() {
     override val rootParser: Parser<SyntaxTree<T>> = this@liftToSyntaxTreeGrammar.rootParser
diff --git a/src/main/kotlin/generated/andFunctions.kt b/src/main/kotlin/generated/andFunctions.kt
index 9f334fa..f017d40 100644
--- a/src/main/kotlin/generated/andFunctions.kt
+++ b/src/main/kotlin/generated/andFunctions.kt
@@ -1,6 +1,6 @@
 package com.github.h0tk3y.betterParse.combinators
-import com.github.h0tk3y.betterParse.parser.Parser
 import com.github.h0tk3y.betterParse.utils.*
+import com.github.h0tk3y.betterParse.parser.*
 
 @JvmName("and2") inline infix fun <reified T1, reified T2, reified T3>
     AndCombinator<Tuple2<T1, T2>>.and(p3: Parser<T3>) =
diff --git a/src/test/kotlin/TestLiftToAst.kt b/src/test/kotlin/TestLiftToAst.kt
index 5952f02..a1abd89 100644
--- a/src/test/kotlin/TestLiftToAst.kt
+++ b/src/test/kotlin/TestLiftToAst.kt
@@ -66,14 +66,14 @@ private fun SyntaxTree<*>.toTopDownStrings() = topDownNodesSequence()
 class TestLiftToAst {
     @Test
     fun continuousRange() {
-        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = true), structureParsers = emptySet())
+        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = true), structureParsers = null)
         val ast = astParser.parseToEnd("a&(b1->c1)|a1&!b|!(a1->a2)->a")
         checkAstContinuousRange(ast)
     }
 
     @Test
     fun astStructure() {
-        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = true), structureParsers = emptySet())
+        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = true), structureParsers = null)
         val ast = astParser.parseToEnd("(!(a)->((b)|(!c))->!(!a)&!(d))")
         val types = ast.toTopDownStrings()
 
@@ -86,7 +86,7 @@ class TestLiftToAst {
 
     @Test
     fun testDropSkipped() {
-        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = false), structureParsers = emptySet())
+        val astParser = booleanGrammar.liftToSyntaxTreeGrammar(LiftToSyntaxTreeOptions(retainSkipped = false), structureParsers = null)
         val ast = astParser.parseToEnd("(!(a)->((b)|(!c))->!(!a)&!(d))")
         val types = ast.toTopDownStrings()