diff --git a/README.md b/README.md
index 2a3c4de6c80..60ee8145ca0 100644
--- a/README.md
+++ b/README.md
@@ -818,8 +818,9 @@ to be `false` and all symbol names will be omitted.
 
 - `unsafe` (default: `false`) — apply "unsafe" transformations (discussion below)
 
-- `unsafe_comps` (default: `false`) — compress expressions like `a <= b` assuming
-  none of the operands can be (coerced to) `NaN`.
+- `unsafe_comps` (default: `false`) — assume operands cannot be (coerced to) `NaN`
+  in numeric comparisons, e.g. `a <= b`. In addition, expressions involving `in`
+  or `instanceof` would never throw.
 
 - `unsafe_Function` (default: `false`) — compress and mangle `Function(args, code)`
   when both `args` and `code` are string literals.
diff --git a/lib/compress.js b/lib/compress.js
index b19bd893b68..fbb6aa2a7dd 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2452,7 +2452,7 @@ Compressor.prototype.compress = function(node) {
 
             function is_last_node(node, parent) {
                 if (node instanceof AST_Await) return true;
-                if (node.TYPE == "Binary") return !can_drop_op(node.operator, node.right);
+                if (node.TYPE == "Binary") return !can_drop_op(node.operator, node.right, compressor);
                 if (node instanceof AST_Call) {
                     var def, fn = node.expression;
                     if (fn instanceof AST_SymbolRef) {
@@ -5551,7 +5551,7 @@ Compressor.prototype.compress = function(node) {
         def(AST_Binary, function(compressor) {
             return this.left.has_side_effects(compressor)
                 || this.right.has_side_effects(compressor)
-                || !can_drop_op(this.operator, this.right);
+                || !can_drop_op(this.operator, this.right, compressor);
         });
         def(AST_Block, function(compressor) {
             return any(this.body, compressor);
@@ -5705,7 +5705,7 @@ Compressor.prototype.compress = function(node) {
         def(AST_Binary, function(compressor) {
             return this.left.may_throw(compressor)
                 || this.right.may_throw(compressor)
-                || !can_drop_op(this.operator, this.right);
+                || !can_drop_op(this.operator, this.right, compressor);
         });
         def(AST_Block, function(compressor) {
             return any(this.body, compressor);
@@ -8533,7 +8533,7 @@ Compressor.prototype.compress = function(node) {
             var left = this.left;
             var right = this.right;
             var op = this.operator;
-            if (!can_drop_op(op, right)) {
+            if (!can_drop_op(op, right, compressor)) {
                 var lhs = left.drop_side_effect_free(compressor, first_in_statement);
                 if (lhs === left) return this;
                 var node = this.clone();
@@ -11202,13 +11202,13 @@ Compressor.prototype.compress = function(node) {
             || node instanceof AST_Object;
     }
 
-    function can_drop_op(op, rhs) {
+    function can_drop_op(op, rhs, compressor) {
         switch (op) {
           case "in":
-            return is_object(rhs);
+            return is_object(rhs) || compressor && compressor.option("unsafe_comps");
           case "instanceof":
             if (rhs instanceof AST_SymbolRef) rhs = rhs.fixed_value();
-            return is_lambda(rhs);
+            return is_lambda(rhs) || compressor && compressor.option("unsafe_comps");
           default:
             return true;
         }
diff --git a/test/compress/comparisons.js b/test/compress/comparisons.js
index 64c75837e57..66602efc81b 100644
--- a/test/compress/comparisons.js
+++ b/test/compress/comparisons.js
@@ -40,6 +40,22 @@ unsafe_comps: {
     }
 }
 
+unsafe_in_instanceof: {
+    options = {
+        side_effects: true,
+        unsafe_comps: true,
+    }
+    input: {
+        var a;
+        42 in a;
+        f() instanceof "foo";
+    }
+    expect: {
+        var a;
+        f();
+    }
+}
+
 dont_change_in_or_instanceof_expressions: {
     input: {
         1 in 1;