diff --git a/lib/compress.js b/lib/compress.js
index 8358e70c5a2..5d97c82ef31 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2566,7 +2566,7 @@ 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, compressor);
+                if (node.TYPE == "Binary") return !can_drop_op(node, compressor);
                 if (node instanceof AST_Call) {
                     var def, fn = node.expression;
                     if (fn instanceof AST_SymbolRef) {
@@ -5820,7 +5820,7 @@ 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, compressor);
+                || !can_drop_op(this, compressor);
         });
         def(AST_Block, function(compressor) {
             return any(this.body, compressor);
@@ -5974,7 +5974,7 @@ 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, compressor);
+                || !can_drop_op(this, compressor);
         });
         def(AST_Block, function(compressor) {
             return any(this.body, compressor);
@@ -6091,7 +6091,7 @@ Compressor.prototype.compress = function(node) {
         def(AST_Binary, function(scope) {
             return this.left.is_constant_expression(scope)
                 && this.right.is_constant_expression(scope)
-                && can_drop_op(this.operator, this.right);
+                && can_drop_op(this);
         });
         def(AST_Class, function(scope) {
             var base = this.extends;
@@ -8813,11 +8813,17 @@ Compressor.prototype.compress = function(node) {
             var left = this.left;
             var right = this.right;
             var op = this.operator;
-            if (!can_drop_op(op, right, compressor)) {
+            if (!can_drop_op(this, compressor)) {
                 var lhs = left.drop_side_effect_free(compressor, first_in_statement);
                 if (lhs === left) return this;
                 var node = this.clone();
-                node.left = lhs || make_node(AST_Number, left, { value: 0 });
+                if (lhs) {
+                    node.left = lhs;
+                } else if (op == "instanceof" && !left.is_constant()) {
+                    node.left = make_node(AST_Array, left, { elements: [] });
+                } else {
+                    node.left = make_node(AST_Number, left, { value: 0 });
+                }
                 return node;
             }
             var rhs = right.drop_side_effect_free(compressor, first_in_statement);
@@ -11566,13 +11572,16 @@ Compressor.prototype.compress = function(node) {
             || node instanceof AST_Object;
     }
 
-    function can_drop_op(op, rhs, compressor) {
-        switch (op) {
+    function can_drop_op(node, compressor) {
+        var rhs = node.right;
+        switch (node.operator) {
           case "in":
             return is_object(rhs) || compressor && compressor.option("unsafe_comps");
           case "instanceof":
             if (rhs instanceof AST_SymbolRef) rhs = rhs.fixed_value();
-            return is_lambda(rhs) || compressor && compressor.option("unsafe_comps");
+            if (rhs instanceof AST_Defun || rhs instanceof AST_Function || is_generator(rhs)) return true;
+            if (is_lambda(rhs) && node.left.is_constant()) return true;
+            return compressor && compressor.option("unsafe_comps");
           default:
             return true;
         }
@@ -12118,8 +12127,10 @@ Compressor.prototype.compress = function(node) {
                 }
                 break;
               case "instanceof":
+                if (!can_drop_op(self, compressor)) break;
                 if (is_lambda(self.right)) return make_sequence(self, [
-                    self,
+                    self.left,
+                    self.right,
                     make_node(AST_False, self),
                 ]).optimize(compressor);
                 break;
diff --git a/test/compress/arrows.js b/test/compress/arrows.js
index 83025c25597..86ac8d494ca 100644
--- a/test/compress/arrows.js
+++ b/test/compress/arrows.js
@@ -690,6 +690,65 @@ inline_iife_within_arrow: {
     node_version: ">=4"
 }
 
+instanceof_lambda_1: {
+    options = {
+        evaluate: true,
+        side_effects: true,
+    }
+    input: {
+        console.log(42 instanceof (() => {}));
+    }
+    expect: {
+        console.log(false);
+    }
+    expect_stdout: "false"
+    node_version: ">=4"
+}
+
+instanceof_lambda_2: {
+    options = {
+        evaluate: true,
+        side_effects: false,
+    }
+    input: {
+        console.log(null instanceof (() => {}));
+    }
+    expect: {
+        console.log((null, () => {}, false));
+    }
+    expect_stdout: "false"
+    node_version: ">=4"
+}
+
+instanceof_lambda_3: {
+    options = {
+        evaluate: true,
+        side_effects: true,
+    }
+    input: {
+        console.log({} instanceof (() => {}));
+    }
+    expect: {
+        console.log({} instanceof (() => {}));
+    }
+    expect_stdout: TypeError("Function has non-object prototype 'undefined' in instanceof check")
+    node_version: ">=4"
+}
+
+instanceof_lambda_4: {
+    options = {
+        side_effects: true,
+    }
+    input: {
+        ({ p: "foo" }) instanceof (() => {});
+    }
+    expect: {
+        [] instanceof (() => {});
+    }
+    expect_stdout: TypeError("Function has non-object prototype 'undefined' in instanceof check")
+    node_version: ">=4"
+}
+
 issue_4388: {
     options = {
         inline: true,
diff --git a/test/compress/awaits.js b/test/compress/awaits.js
index f1d3628ccdb..42e86c8033a 100644
--- a/test/compress/awaits.js
+++ b/test/compress/awaits.js
@@ -1348,7 +1348,7 @@ functions_inner_var: {
     node_version: ">=8"
 }
 
-instanceof_lambda: {
+instanceof_lambda_1: {
     options = {
         evaluate: true,
         side_effects: true,
@@ -1363,6 +1363,50 @@ instanceof_lambda: {
     node_version: ">=8"
 }
 
+instanceof_lambda_2: {
+    options = {
+        evaluate: true,
+        side_effects: false,
+    }
+    input: {
+        console.log(null instanceof async function() {});
+    }
+    expect: {
+        console.log((null, async function() {}, false));
+    }
+    expect_stdout: "false"
+    node_version: ">=8"
+}
+
+instanceof_lambda_3: {
+    options = {
+        evaluate: true,
+        side_effects: true,
+    }
+    input: {
+        console.log({} instanceof async function() {});
+    }
+    expect: {
+        console.log({} instanceof async function() {});
+    }
+    expect_stdout: TypeError("Function has non-object prototype 'undefined' in instanceof check")
+    node_version: ">=8"
+}
+
+instanceof_lambda_4: {
+    options = {
+        side_effects: true,
+    }
+    input: {
+        ({ p: "foo" }) instanceof async function() {};
+    }
+    expect: {
+        [] instanceof async function() {};
+    }
+    expect_stdout: TypeError("Function has non-object prototype 'undefined' in instanceof check")
+    node_version: ">=8"
+}
+
 issue_4335_1: {
     options = {
         inline: true,
diff --git a/test/compress/pure_funcs.js b/test/compress/pure_funcs.js
index 688a105719f..76f57cd9c4e 100644
--- a/test/compress/pure_funcs.js
+++ b/test/compress/pure_funcs.js
@@ -147,7 +147,7 @@ relational: {
         "bar" >= "bar";
     }
     expect: {
-        0 instanceof bar();
+        [] instanceof bar();
         bar();
         bar(), bar();
         bar();