diff --git a/js/js.tests/test/org/jetbrains/k2js/test/semantics/PatternMatchingTest.java b/js/js.tests/test/org/jetbrains/k2js/test/semantics/PatternMatchingTest.java
index f64f41cf244..6a6759331c3 100644
--- a/js/js.tests/test/org/jetbrains/k2js/test/semantics/PatternMatchingTest.java
+++ b/js/js.tests/test/org/jetbrains/k2js/test/semantics/PatternMatchingTest.java
@@ -72,7 +72,16 @@ public final class PatternMatchingTest extends SingleFileTranslationTest {
         try {
             checkFooBoxIsTrue("whenAsExpressionWithThrow.kt");
             fail();
-        } catch (JavaScriptException e) {
+        }
+        catch (JavaScriptException e) {
         }
     }
+
+    public void testKT1665() throws Exception {
+        checkOutput("kt1665.kt", "a", "");
+    }
+
+    public void testWhenWithoutExpression() throws Exception {
+        checkFooBoxIsTrue("whenWithoutExpression.kt");
+    }
 }
\ No newline at end of file
diff --git a/js/js.translator/src/org/jetbrains/k2js/translate/expression/PatternTranslator.java b/js/js.translator/src/org/jetbrains/k2js/translate/expression/PatternTranslator.java
index fdd967fee03..e54d42c7ad5 100644
--- a/js/js.translator/src/org/jetbrains/k2js/translate/expression/PatternTranslator.java
+++ b/js/js.translator/src/org/jetbrains/k2js/translate/expression/PatternTranslator.java
@@ -61,12 +61,16 @@ public final class PatternTranslator extends AbstractTranslator {
     }
 
     @NotNull
-    public JsExpression translatePattern(@NotNull JetPattern pattern, @NotNull JsExpression expressionToMatch) {
+    public JsExpression translatePattern(@NotNull JetPattern pattern, @Nullable JsExpression expressionToMatch) {
+        if (expressionToMatch == null) {
+            assert pattern instanceof JetExpressionPattern : "When using when without parameters we can have only expression patterns";
+            return translateExpressionForExpressionPattern((JetExpressionPattern)pattern);
+        }
         if (pattern instanceof JetTypePattern) {
-            return translateTypePattern(expressionToMatch, (JetTypePattern) pattern);
+            return translateTypePattern(expressionToMatch, (JetTypePattern)pattern);
         }
         if (pattern instanceof JetExpressionPattern) {
-            return translateExpressionPattern(expressionToMatch, (JetExpressionPattern) pattern);
+            return translateExpressionPattern(expressionToMatch, (JetExpressionPattern)pattern);
         }
         throw new AssertionError("Unsupported pattern type " + pattern.getClass());
     }
@@ -124,16 +128,20 @@ public final class PatternTranslator extends AbstractTranslator {
     @NotNull
     private JsNameRef getClassNameReferenceForTypeReference(@NotNull JetTypeReference typeReference) {
         ClassDescriptor referencedClass = BindingUtils.getClassDescriptorForTypeReference
-                (bindingContext(), typeReference);
+            (bindingContext(), typeReference);
         return TranslationUtils.getQualifiedReference(context(), referencedClass);
     }
 
     @NotNull
-    private JsExpression translateExpressionPattern(JsExpression expressionToMatch, JetExpressionPattern pattern) {
-        JetExpression patternExpression = pattern.getExpression();
-        assert patternExpression != null : "Expression patter should have an expression.";
-        JsExpression expressionToMatchAgainst =
-                Translation.translateAsExpression(patternExpression, context());
+    private JsExpression translateExpressionPattern(@NotNull JsExpression expressionToMatch, JetExpressionPattern pattern) {
+        JsExpression expressionToMatchAgainst = translateExpressionForExpressionPattern(pattern);
         return equality(expressionToMatch, expressionToMatchAgainst);
     }
+
+    @NotNull
+    private JsExpression translateExpressionForExpressionPattern(@NotNull JetExpressionPattern pattern) {
+        JetExpression patternExpression = pattern.getExpression();
+        assert patternExpression != null : "Expression patter should have an expression.";
+        return Translation.translateAsExpression(patternExpression, context());
+    }
 }
diff --git a/js/js.translator/src/org/jetbrains/k2js/translate/expression/WhenTranslator.java b/js/js.translator/src/org/jetbrains/k2js/translate/expression/WhenTranslator.java
index a97b04ebf29..3ae42af63d9 100644
--- a/js/js.translator/src/org/jetbrains/k2js/translate/expression/WhenTranslator.java
+++ b/js/js.translator/src/org/jetbrains/k2js/translate/expression/WhenTranslator.java
@@ -36,17 +36,17 @@ import static org.jetbrains.k2js.translate.utils.JsAstUtils.*;
 /**
  * @author Pavel Talanov
  */
-public class WhenTranslator extends AbstractTranslator {
+public final class WhenTranslator extends AbstractTranslator {
 
     @NotNull
-    static public JsNode translateWhenExpression(@NotNull JetWhenExpression expression, @NotNull TranslationContext context) {
+    public static JsNode translateWhenExpression(@NotNull JetWhenExpression expression, @NotNull TranslationContext context) {
         WhenTranslator translator = new WhenTranslator(expression, context);
         return translator.translate();
     }
 
     @NotNull
     private final JetWhenExpression whenExpression;
-    @NotNull
+    @Nullable
     private final JsExpression expressionToMatch;
     @NotNull
     private final TemporaryVariable dummyCounter;
@@ -179,7 +179,7 @@ public class WhenTranslator extends AbstractTranslator {
     @NotNull
     private JsExpression translatePatternCondition(@NotNull JetWhenCondition condition) {
         JsExpression patternMatchExpression = Translation.patternTranslator(context()).
-                translatePattern(getPattern(condition), expressionToMatch);
+            translatePattern(getPattern(condition), expressionToMatch);
         if (isNegated(condition)) {
             return negated(patternMatchExpression);
         }
@@ -188,7 +188,7 @@ public class WhenTranslator extends AbstractTranslator {
 
     private static boolean isNegated(@NotNull JetWhenCondition condition) {
         if (condition instanceof JetWhenConditionIsPattern) {
-            return ((JetWhenConditionIsPattern) condition).isNegated();
+            return ((JetWhenConditionIsPattern)condition).isNegated();
         }
         return false;
     }
@@ -197,10 +197,10 @@ public class WhenTranslator extends AbstractTranslator {
     private static JetPattern getPattern(@NotNull JetWhenCondition condition) {
         JetPattern pattern;
         if (condition instanceof JetWhenConditionIsPattern) {
-            pattern = ((JetWhenConditionIsPattern) condition).getPattern();
+            pattern = ((JetWhenConditionIsPattern)condition).getPattern();
         }
         else if (condition instanceof JetWhenConditionWithExpression) {
-            pattern = ((JetWhenConditionWithExpression) condition).getPattern();
+            pattern = ((JetWhenConditionWithExpression)condition).getPattern();
         }
         else {
             throw new AssertionError("Wrong type of JetWhenCondition");
@@ -209,10 +209,12 @@ public class WhenTranslator extends AbstractTranslator {
         return pattern;
     }
 
-    @NotNull
+    @Nullable
     private JsExpression translateExpressionToMatch(@NotNull JetWhenExpression expression) {
         JetExpression subject = expression.getSubjectExpression();
-        assert subject != null : "Subject should not be null.";
+        if (subject == null) {
+            return null;
+        }
         return Translation.translateAsExpression(subject, context());
     }
 }
diff --git a/js/js.translator/testFiles/patternMatching/cases/kt1665.kt b/js/js.translator/testFiles/patternMatching/cases/kt1665.kt
new file mode 100644
index 00000000000..1d0721b5179
--- /dev/null
+++ b/js/js.translator/testFiles/patternMatching/cases/kt1665.kt
@@ -0,0 +1,9 @@
+fun main(args : Array<String>) {
+  val a = 10
+  val b = 3
+  when {
+    a > b -> println("a")
+    b > a  -> println("b")
+    else -> println("Unknown")
+  }
+}
\ No newline at end of file
diff --git a/js/js.translator/testFiles/patternMatching/cases/whenWithoutExpression.kt b/js/js.translator/testFiles/patternMatching/cases/whenWithoutExpression.kt
new file mode 100644
index 00000000000..b8d933a30ce
--- /dev/null
+++ b/js/js.translator/testFiles/patternMatching/cases/whenWithoutExpression.kt
@@ -0,0 +1,6 @@
+package foo
+
+fun box() = when {
+  1 > 3 -> false
+  else -> true
+}
\ No newline at end of file