diff --git a/lib/compress.js b/lib/compress.js
index c34c76301e..f7c0af30e9 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1200,7 +1200,7 @@ merge(Compressor.prototype, {
                 if (!hit) {
                     if (node !== hit_stack[hit_index]) return node;
                     hit_index++;
-                    if (hit_index < hit_stack.length) return handle_custom_scan_order(node);
+                    if (hit_index < hit_stack.length) return handle_custom_scan_order(node, scanner);
                     hit = true;
                     stop_after = (value_def ? find_stop_value : find_stop)(node, 0);
                     if (stop_after === node) abort = true;
@@ -1279,7 +1279,7 @@ merge(Compressor.prototype, {
                     can_replace = replace;
                     return node;
                 }
-                return handle_custom_scan_order(node);
+                return handle_custom_scan_order(node, scanner);
             }, function(node) {
                 if (abort) return;
                 if (stop_after === node) abort = true;
@@ -1398,12 +1398,12 @@ merge(Compressor.prototype, {
                 }
             }
 
-            function handle_custom_scan_order(node) {
+            function handle_custom_scan_order(node, tt) {
                 // Skip (non-executed) functions
                 if (node instanceof AST_Scope) return node;
                 // Scan case expressions first in a switch statement
                 if (node instanceof AST_Switch) {
-                    node.expression = node.expression.transform(scanner);
+                    node.expression = node.expression.transform(tt);
                     for (var i = 0; !abort && i < node.body.length; i++) {
                         var branch = node.body[i];
                         if (branch instanceof AST_Case) {
@@ -1411,7 +1411,7 @@ merge(Compressor.prototype, {
                                 if (branch !== hit_stack[hit_index]) continue;
                                 hit_index++;
                             }
-                            branch.expression = branch.expression.transform(scanner);
+                            branch.expression = branch.expression.transform(tt);
                             if (!replace_all) break;
                             scan_rhs = false;
                         }
@@ -1930,12 +1930,14 @@ merge(Compressor.prototype, {
                     });
                     return true;
                 }
-                var found = false;
-                return statements[stat_index].transform(new TreeTransformer(function(node, descend, in_list) {
-                    if (found) return node;
-                    if (node instanceof AST_Scope) return node;
-                    if (node !== expr && node.body !== expr) return;
-                    found = true;
+                var end = hit_stack.length - 1;
+                if (hit_stack[end - 1].body === hit_stack[end]) end--;
+                var tt = new TreeTransformer(function(node, descend, in_list) {
+                    if (hit) return node;
+                    if (node !== hit_stack[hit_index]) return node;
+                    hit_index++;
+                    if (hit_index <= end) return handle_custom_scan_order(node, tt);
+                    hit = true;
                     if (node instanceof AST_VarDef) {
                         node.value = null;
                         declare_only[node.name.name] = (declare_only[node.name.name] || 0) + 1;
@@ -1943,7 +1945,11 @@ merge(Compressor.prototype, {
                         return node;
                     }
                     return in_list ? List.skip : null;
-                }, patch_sequence));
+                }, patch_sequence);
+                abort = false;
+                hit = false;
+                hit_index = 0;
+                return statements[stat_index].transform(tt);
             }
 
             function patch_sequence(node) {
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index ff0837a2ec..dee370cf36 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -7979,3 +7979,39 @@ issue_3884: {
     }
     expect_stdout: "101 32"
 }
+
+issue_3891: {
+    options = {
+        collapse_vars: true,
+        passes: 2,
+        reduce_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        function log(a) {
+            console.log(typeof a);
+        }
+        log(function f() {
+            try {
+                do {
+                    var b = function() {}();
+                } while (f = 0, b.p);
+            } catch (e) {
+                var f;
+                b;
+            }
+        });
+    }
+    expect: {
+        function log(a) {
+            console.log(typeof a);
+        }
+        log(function() {
+            try {
+                do {} while ((void 0).p);
+            } catch (e) {}
+        });
+    }
+    expect_stdout: "function"
+}