diff --git a/lib/compress.js b/lib/compress.js
index 5bb9bcb0..c911de81 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -6412,7 +6412,7 @@ Compressor.prototype.compress = function(node) {
         Object.keys(assign_in_use).forEach(function(id) {
             var assigns = assign_in_use[id];
             if (!assigns) {
-                delete assign_in_use[id];
+                assign_in_use[id] = undefined;
                 return;
             }
             assigns = assigns.reduce(function(in_use, assigns) {
@@ -6433,7 +6433,7 @@ Compressor.prototype.compress = function(node) {
             if (assigns.length == in_use.length) {
                 assign_in_use[id] = in_use;
             } else {
-                delete assign_in_use[id];
+                assign_in_use[id] = undefined;
             }
         });
         // pass 3: we should drop declarations not in_use
@@ -6834,10 +6834,10 @@ Compressor.prototype.compress = function(node) {
                     }
                   case 1:
                     if (tail.length == 0) {
-                        var id = head[0].name.definition().id;
-                        if (id in for_ins) {
+                        var for_in = for_ins[head[0].name.definition().id];
+                        if (for_in) {
                             node.definitions = head;
-                            for_ins[id].init = node;
+                            for_in.init = node;
                             break;
                         }
                     }
@@ -6932,7 +6932,7 @@ Compressor.prototype.compress = function(node) {
                 var def = sym.definition();
                 if (def.id in in_use_ids) return;
                 log(sym, "Dropping unused loop variable {name}");
-                if (for_ins[def.id] === node) delete for_ins[def.id];
+                if (for_ins[def.id] === node) for_ins[def.id] = undefined;
                 var body = [];
                 var value = node.object.drop_side_effect_free(compressor);
                 if (value) {
@@ -7115,7 +7115,7 @@ Compressor.prototype.compress = function(node) {
             if (node instanceof AST_ForIn) {
                 if (node.init instanceof AST_SymbolRef && scope === self) {
                     var id = node.init.definition().id;
-                    if (!(id in for_ins)) for_ins[id] = node;
+                    if (!for_ins[id]) for_ins[id] = node;
                 }
                 if (!drop_vars || !compressor.option("loops")) return;
                 if (!is_empty(node.body)) return;