diff --git a/lib/compress.js b/lib/compress.js
index 6f102e7fae..4c9026d179 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2560,7 +2560,7 @@ merge(Compressor.prototype, {
                 if (def.undeclared) return;
                 if (is_arguments(def)) return;
                 if (value !== rhs) {
-                    if (value.is_immutable()) return;
+                    if (is_lhs_read_only(value, compressor)) return;
                     var referenced = def.references.length - def.replaced;
                     if (referenced < 2) return;
                     candidate = candidate.clone();
@@ -7330,8 +7330,9 @@ merge(Compressor.prototype, {
         def(AST_Assign, function(compressor) {
             var left = this.left;
             if (left instanceof AST_PropAccess) {
-                if (left.expression.may_throw_on_access(compressor, true)) return this;
-                if (compressor.has_directive("use strict") && left.expression.is_constant()) return this;
+                var expr = left.expression;
+                if (expr.may_throw_on_access(compressor, true)) return this;
+                if (compressor.has_directive("use strict") && expr.is_constant()) return this;
             }
             if (left.has_side_effects(compressor)) return this;
             var right = this.right;
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index 451681b67d..4051bb1489 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -8993,3 +8993,20 @@ issue_4852: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4865: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var NaN;
+        var a = NaN = "PASS";
+        console.log(a, NaN);
+    }
+    expect: {
+        var NaN;
+        var a = NaN = "PASS";
+        console.log(a, NaN);
+    }
+    expect_stdout: true
+}