diff --git a/README.md b/README.md
index b3f75bb..1bc1ffd 100644
--- a/README.md
+++ b/README.md
@@ -66,6 +66,8 @@ class DrinkSodaTest(
 }
 ```
 
+If the parameter is nullable, Burst will also test with null.
+
 ### Multiple Parameters
 
 Use multiple parameters to test all variations.
diff --git a/burst-kotlin-plugin-tests/src/test/kotlin/app/cash/burst/kotlin/BurstKotlinPluginTest.kt b/burst-kotlin-plugin-tests/src/test/kotlin/app/cash/burst/kotlin/BurstKotlinPluginTest.kt
index dbb0642..e2efeb7 100644
--- a/burst-kotlin-plugin-tests/src/test/kotlin/app/cash/burst/kotlin/BurstKotlinPluginTest.kt
+++ b/burst-kotlin-plugin-tests/src/test/kotlin/app/cash/burst/kotlin/BurstKotlinPluginTest.kt
@@ -106,7 +106,7 @@ class BurstKotlinPluginTest {
     assertEquals(KotlinCompilation.ExitCode.COMPILATION_ERROR, result.exitCode, result.messages)
     assertThat(result.messages).contains(
       "CoffeeTest.kt:7:12 " +
-        "@Burst parameter must be a boolean, enum, or have a burstValues() default value",
+        "@Burst parameter must be a boolean, an enum, or have a burstValues() default value",
     )
   }
 
@@ -135,7 +135,7 @@ class BurstKotlinPluginTest {
     assertEquals(KotlinCompilation.ExitCode.COMPILATION_ERROR, result.exitCode, result.messages)
     assertThat(result.messages).contains(
       "CoffeeTest.kt:9:12 " +
-        "@Burst parameter default value must be burstValues(), a constant, or absent",
+        "@Burst parameter default must be burstValues(), a constant, null, or absent",
     )
   }
 
@@ -681,6 +681,166 @@ class BurstKotlinPluginTest {
     )
   }
 
+  @Test
+  fun nullableEnumNoDefault() {
+    val result = compile(
+      sourceFile = SourceFile.kotlin(
+        "CoffeeTest.kt",
+        """
+        import app.cash.burst.Burst
+        import kotlin.test.Test
+
+        @Burst
+        class CoffeeTest {
+          val log = mutableListOf<String>()
+
+          @Test
+          fun test(espresso: Espresso?) {
+            log += "running ${'$'}espresso"
+          }
+        }
+
+        enum class Espresso { Decaf, Regular, Double }
+        """,
+      ),
+    )
+    assertEquals(KotlinCompilation.ExitCode.OK, result.exitCode, result.messages)
+
+    val baseClass = result.classLoader.loadClass("CoffeeTest")
+    val baseInstance = baseClass.constructors.single().newInstance()
+    val baseLog = baseClass.getMethod("getLog").invoke(baseInstance) as MutableList<*>
+
+    baseClass.getMethod("test_Decaf").invoke(baseInstance)
+    baseClass.getMethod("test_Regular").invoke(baseInstance)
+    baseClass.getMethod("test_Double").invoke(baseInstance)
+    baseClass.getMethod("test_null").invoke(baseInstance)
+    assertThat(baseLog).containsExactly(
+      "running Decaf",
+      "running Regular",
+      "running Double",
+      "running null",
+    )
+  }
+
+  @Test
+  fun nullableBooleanNoDefault() {
+    val result = compile(
+      sourceFile = SourceFile.kotlin(
+        "CoffeeTest.kt",
+        """
+        import app.cash.burst.Burst
+        import kotlin.test.Test
+
+        @Burst
+        class CoffeeTest {
+          val log = mutableListOf<String>()
+
+          @Test
+          fun test(iced: Boolean?) {
+            log += "running ${'$'}iced"
+          }
+        }
+
+        enum class Espresso { Decaf, Regular, Double }
+        """,
+      ),
+    )
+    assertEquals(KotlinCompilation.ExitCode.OK, result.exitCode, result.messages)
+
+    val baseClass = result.classLoader.loadClass("CoffeeTest")
+    val baseInstance = baseClass.constructors.single().newInstance()
+    val baseLog = baseClass.getMethod("getLog").invoke(baseInstance) as MutableList<*>
+
+    baseClass.getMethod("test_false").invoke(baseInstance)
+    baseClass.getMethod("test_true").invoke(baseInstance)
+    baseClass.getMethod("test_null").invoke(baseInstance)
+    assertThat(baseLog).containsExactly(
+      "running false",
+      "running true",
+      "running null",
+    )
+  }
+
+  @Test
+  fun nullableEnumAsDefault() {
+    val result = compile(
+      sourceFile = SourceFile.kotlin(
+        "CoffeeTest.kt",
+        """
+        import app.cash.burst.Burst
+        import kotlin.test.Test
+
+        @Burst
+        class CoffeeTest {
+          val log = mutableListOf<String>()
+
+          @Test
+          fun test(espresso: Espresso? = null) {
+            log += "running ${'$'}espresso"
+          }
+        }
+
+        enum class Espresso { Decaf, Regular, Double }
+        """,
+      ),
+    )
+    assertEquals(KotlinCompilation.ExitCode.OK, result.exitCode, result.messages)
+
+    val baseClass = result.classLoader.loadClass("CoffeeTest")
+    val baseInstance = baseClass.constructors.single().newInstance()
+    val baseLog = baseClass.getMethod("getLog").invoke(baseInstance) as MutableList<*>
+
+    baseClass.getMethod("test_Decaf").invoke(baseInstance)
+    baseClass.getMethod("test_Regular").invoke(baseInstance)
+    baseClass.getMethod("test_Double").invoke(baseInstance)
+    baseClass.getMethod("test").invoke(baseInstance)
+    assertThat(baseLog).containsExactly(
+      "running Decaf",
+      "running Regular",
+      "running Double",
+      "running null",
+    )
+  }
+
+  @Test
+  fun nullableBooleanAsDefault() {
+    val result = compile(
+      sourceFile = SourceFile.kotlin(
+        "CoffeeTest.kt",
+        """
+        import app.cash.burst.Burst
+        import kotlin.test.Test
+
+        @Burst
+        class CoffeeTest {
+          val log = mutableListOf<String>()
+
+          @Test
+          fun test(iced: Boolean? = null) {
+            log += "running ${'$'}iced"
+          }
+        }
+
+        enum class Espresso { Decaf, Regular, Double }
+        """,
+      ),
+    )
+    assertEquals(KotlinCompilation.ExitCode.OK, result.exitCode, result.messages)
+
+    val baseClass = result.classLoader.loadClass("CoffeeTest")
+    val baseInstance = baseClass.constructors.single().newInstance()
+    val baseLog = baseClass.getMethod("getLog").invoke(baseInstance) as MutableList<*>
+
+    baseClass.getMethod("test_false").invoke(baseInstance)
+    baseClass.getMethod("test_true").invoke(baseInstance)
+    baseClass.getMethod("test").invoke(baseInstance)
+    assertThat(baseLog).containsExactly(
+      "running false",
+      "running true",
+      "running null",
+    )
+  }
+
   private val Class<*>.testSuffixes: List<String>
     get() = methods.mapNotNull {
       when {
diff --git a/burst-kotlin-plugin/src/main/kotlin/app/cash/burst/kotlin/Argument.kt b/burst-kotlin-plugin/src/main/kotlin/app/cash/burst/kotlin/Argument.kt
index 3057617..3aaeb2a 100644
--- a/burst-kotlin-plugin/src/main/kotlin/app/cash/burst/kotlin/Argument.kt
+++ b/burst-kotlin-plugin/src/main/kotlin/app/cash/burst/kotlin/Argument.kt
@@ -33,6 +33,7 @@ import org.jetbrains.kotlin.ir.symbols.UnsafeDuringIrConstructionAPI
 import org.jetbrains.kotlin.ir.types.IrType
 import org.jetbrains.kotlin.ir.types.classFqName
 import org.jetbrains.kotlin.ir.types.getClass
+import org.jetbrains.kotlin.ir.types.isNullable
 import org.jetbrains.kotlin.ir.util.classId
 import org.jetbrains.kotlin.ir.util.deepCopyWithSymbols
 import org.jetbrains.kotlin.ir.util.defaultType
@@ -86,6 +87,20 @@ private class BooleanArgument(
   }
 }
 
+private class NullArgument(
+  private val original: IrElement,
+  private val type: IrType,
+  override val isDefault: Boolean,
+) : Argument {
+  override val name = "null"
+
+  override fun expression() = IrConstImpl.constNull(original.startOffset, original.endOffset, type)
+
+  override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R {
+    return original.accept(visitor, data)
+  }
+}
+
 @UnsafeDuringIrConstructionAPI
 private class BurstValuesArgument(
   private val parameter: IrValueParameter,
@@ -192,49 +207,86 @@ private fun IrExpression.suggestedName(): String? {
 private fun enumValueArguments(
   referenceClass: IrClass,
   parameter: IrValueParameter,
-): List<EnumValueArgument> {
+): List<Argument> {
   val enumEntries = referenceClass.declarations.filterIsInstance<IrEnumEntry>()
-  val defaultValueSymbol = parameter.defaultValue?.let { defaultValue ->
-    (defaultValue.expression as? IrGetEnumValue)?.symbol ?: unexpectedDefaultValue(parameter)
+  val hasDefaultValue = parameter.defaultValue != null
+  val defaultEnumSymbol = parameter.defaultValue?.let { defaultValue ->
+    val expression = defaultValue.expression
+    when {
+      expression is IrGetEnumValue -> expression.symbol
+      expression is IrConst<*> && expression.value == null -> null
+      else -> unexpectedDefaultValue(parameter)
+    }
   }
 
-  return enumEntries.map {
-    EnumValueArgument(
-      original = parameter,
-      type = parameter.type,
-      isDefault = it.symbol == defaultValueSymbol,
-      value = it,
-    )
+  return buildList {
+    for (enumEntry in enumEntries) {
+      add(
+        EnumValueArgument(
+          original = parameter,
+          type = parameter.type,
+          isDefault = hasDefaultValue && enumEntry.symbol == defaultEnumSymbol,
+          value = enumEntry,
+        ),
+      )
+    }
+    if (parameter.type.isNullable()) {
+      add(
+        NullArgument(
+          original = parameter,
+          type = parameter.type,
+          isDefault = hasDefaultValue && defaultEnumSymbol == null,
+        ),
+      )
+    }
   }
 }
 
 private fun IrPluginContext.booleanArguments(
   parameter: IrValueParameter,
-): List<BooleanArgument> {
+): List<Argument> {
+  val hasDefaultValue = parameter.defaultValue != null
   val defaultValue = parameter.defaultValue?.let { defaultValue ->
-    (defaultValue.expression as? IrConst<*>)?.value ?: unexpectedDefaultValue(parameter)
+    val expression = defaultValue.expression
+    when {
+      expression is IrConst<*> -> expression.value
+      else -> unexpectedDefaultValue(parameter)
+    }
   }
 
-  return listOf(false, true).map {
-    BooleanArgument(
-      original = parameter,
-      booleanType = irBuiltIns.booleanType,
-      isDefault = defaultValue == it,
-      value = it,
-    )
+  return buildList {
+    for (b in listOf(false, true)) {
+      add(
+        BooleanArgument(
+          original = parameter,
+          booleanType = irBuiltIns.booleanType,
+          isDefault = hasDefaultValue && defaultValue == b,
+          value = b,
+        ),
+      )
+    }
+    if (parameter.type.isNullable()) {
+      add(
+        NullArgument(
+          original = parameter,
+          type = parameter.type,
+          isDefault = hasDefaultValue && defaultValue == null,
+        ),
+      )
+    }
   }
 }
 
 private fun unexpectedParameter(parameter: IrValueParameter): Nothing {
   throw BurstCompilationException(
-    "@Burst parameter must be a boolean, enum, or have a burstValues() default value",
+    "@Burst parameter must be a boolean, an enum, or have a burstValues() default value",
     parameter,
   )
 }
 
 private fun unexpectedDefaultValue(parameter: IrValueParameter): Nothing {
   throw BurstCompilationException(
-    "@Burst parameter default value must be burstValues(), a constant, or absent",
+    "@Burst parameter default must be burstValues(), a constant, null, or absent",
     parameter,
   )
 }