diff --git a/compiler/backend/src/org/jetbrains/jet/codegen/optimization/boxing/BoxingInterpreter.java b/compiler/backend/src/org/jetbrains/jet/codegen/optimization/boxing/BoxingInterpreter.java
index 32b01ebc60c..47cd1bc2ee3 100644
--- a/compiler/backend/src/org/jetbrains/jet/codegen/optimization/boxing/BoxingInterpreter.java
+++ b/compiler/backend/src/org/jetbrains/jet/codegen/optimization/boxing/BoxingInterpreter.java
@@ -221,12 +221,12 @@ public class BoxingInterpreter extends OptimizationBasicInterpreter {
 
         if (v instanceof BoxedBasicValue) {
             onMergeFail((BoxedBasicValue) v);
-            v = new BasicValue(v.getType());
+            return MIXED_VALUE;
         }
 
         if (w instanceof BoxedBasicValue) {
             onMergeFail((BoxedBasicValue) w);
-            w = new BasicValue(w.getType());
+            return MIXED_VALUE;
         }
 
         return super.merge(v, w);
diff --git a/compiler/backend/src/org/jetbrains/jet/codegen/optimization/common/OptimizationBasicInterpreter.java b/compiler/backend/src/org/jetbrains/jet/codegen/optimization/common/OptimizationBasicInterpreter.java
index a1eda689cc2..0876b79afb0 100644
--- a/compiler/backend/src/org/jetbrains/jet/codegen/optimization/common/OptimizationBasicInterpreter.java
+++ b/compiler/backend/src/org/jetbrains/jet/codegen/optimization/common/OptimizationBasicInterpreter.java
@@ -27,7 +27,7 @@ import org.jetbrains.org.objectweb.asm.tree.analysis.BasicInterpreter;
 import org.jetbrains.org.objectweb.asm.tree.analysis.BasicValue;
 
 public class OptimizationBasicInterpreter extends BasicInterpreter {
-    private static final BasicValue MIXED_VALUE = new BasicValue(Type.getObjectType("#"));
+    public static final BasicValue MIXED_VALUE = new BasicValue(Type.getObjectType("#"));
     private static final BasicValue BOOLEAN_VALUE = new BasicValue(Type.BOOLEAN_TYPE);
     private static final BasicValue CHAR_VALUE = new BasicValue(Type.CHAR_TYPE);
     private static final BasicValue BYTE_VALUE = new BasicValue(Type.BYTE_TYPE);
diff --git a/compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5588.kt b/compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5588.kt
new file mode 100644
index 00000000000..ece5914c504
--- /dev/null
+++ b/compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5588.kt
@@ -0,0 +1,10 @@
+fun box() : String {
+    val s = "notA"
+    val id = when (s) {
+        "a" -> 1
+        else -> null
+    }
+
+    if (id == null) return "OK"
+    return "fail"
+}
diff --git a/compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java b/compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java
index 5b373bb94bd..53573a33133 100644
--- a/compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java
+++ b/compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java
@@ -134,6 +134,11 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
             doTestWithStdlib("compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5493.kt");
         }
         
+        @TestMetadata("kt5588.kt")
+        public void testKt5588() throws Exception {
+            doTestWithStdlib("compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5588.kt");
+        }
+        
         @TestMetadata("nullCheck.kt")
         public void testNullCheck() throws Exception {
             doTestWithStdlib("compiler/testData/codegen/boxWithStdlib/boxingOptimization/nullCheck.kt");