diff --git a/lib/compress.js b/lib/compress.js
index 9245dccc..e5d64c9a 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3055,8 +3055,9 @@ merge(Compressor.prototype, {
             if (this.expression instanceof AST_Seq) {
                 var seq = this.expression;
                 var x = seq.to_array();
-                this.expression = x.pop();
-                x.push(this);
+                var e = this.clone();
+                e.expression = x.pop();
+                x.push(e);
                 seq = AST_Seq.from_array(x).transform(compressor);
                 return seq;
             }
@@ -3110,9 +3111,14 @@ merge(Compressor.prototype, {
         if (e instanceof AST_Binary
             && (self.operator == "+" || self.operator == "-")
             && (e.operator == "*" || e.operator == "/" || e.operator == "%")) {
-            self.expression = e.left;
-            e.left = self;
-            return e;
+            return make_node(AST_Binary, self, {
+                operator: e.operator,
+                left: make_node(AST_UnaryPrefix, e.left, {
+                    operator: self.operator,
+                    expression: e.left
+                }),
+                right: e.right
+            });
         }
         // avoids infinite recursion of numerals
         if (self.operator != "-"
@@ -3131,23 +3137,25 @@ merge(Compressor.prototype, {
             if (this.left instanceof AST_Seq) {
                 var seq = this.left;
                 var x = seq.to_array();
-                this.left = x.pop();
-                x.push(this);
+                var e = this.clone();
+                e.left = x.pop();
+                x.push(e);
                 return AST_Seq.from_array(x).optimize(compressor);
             }
             if (this.right instanceof AST_Seq && !this.left.has_side_effects(compressor)) {
                 var assign = this.operator == "=" && this.left instanceof AST_SymbolRef;
-                var root = this.right;
+                var root = this.right.clone();
                 var cursor, seq = root;
                 while (assign || !seq.car.has_side_effects(compressor)) {
                     cursor = seq;
                     if (seq.cdr instanceof AST_Seq) {
-                        seq = seq.cdr;
+                        seq = seq.cdr = seq.cdr.clone();
                     } else break;
                 }
                 if (cursor) {
-                    this.right = cursor.cdr;
-                    cursor.cdr = this;
+                    var e = this.clone();
+                    e.right = cursor.cdr;
+                    cursor.cdr = e;
                     return root.optimize(compressor);
                 }
             }
@@ -3558,7 +3566,7 @@ merge(Compressor.prototype, {
                     }
                 }
                 if (d.should_replace) {
-                    return best_of_expression(d.should_replace.clone(true).optimize(compressor), fixed.clone(true));
+                    return best_of_expression(d.should_replace.optimize(compressor), fixed).clone(true);
                 }
             }
         }
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index f5b16ebe..5ad5c6da 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -1897,3 +1897,22 @@ booleans: {
     }
     expect_stdout: "PASS"
 }
+
+side_effects_assign: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        var a = typeof void (a && a.in == 1, 0);
+        console.log(a);
+    }
+    expect: {
+        var a = typeof void (a && a.in);
+        console.log(a);
+    }
+    expect_stdout: "undefined"
+}