diff --git a/lib/compress.js b/lib/compress.js
index 99543fc984e..c7b1b792f01 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -12865,19 +12865,17 @@ Compressor.prototype.compress = function(node) {
         var alt_tail = alternative.tail_node();
         // x ? y : y ---> x, y
         // x ? (a, c) : (b, c) ---> x ? a : b, c
-        if (seq_tail.equals(alt_tail)) {
-            return make_sequence(self, consequent.equals(alternative) ? [
-                condition,
-                consequent,
-            ] : [
-                make_node(AST_Conditional, self, {
-                    condition: condition,
-                    consequent: pop_seq(consequent),
-                    alternative: pop_seq(alternative),
-                }),
-                seq_tail,
-            ]).optimize(compressor);
-        }
+        if (seq_tail.equals(alt_tail)) return make_sequence(self, consequent.equals(alternative) ? [
+            condition,
+            consequent,
+        ] : [
+            make_node(AST_Conditional, self, {
+                condition: condition,
+                consequent: pop_seq(consequent),
+                alternative: pop_seq(alternative),
+            }),
+            alt_tail,
+        ]).optimize(compressor);
         // x ? y.p : z.p ---> (x ? y : z).p
         // x ? y(a) : z(a) ---> (x ? y : z)(a)
         // x ? y.f(a) : z.f(a) ---> (x ? y : z).f(a)
diff --git a/test/compress/conditionals.js b/test/compress/conditionals.js
index dc035bb14b9..72a1c2ec116 100644
--- a/test/compress/conditionals.js
+++ b/test/compress/conditionals.js
@@ -2878,3 +2878,53 @@ issue_5546_3: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5666_1: {
+    options = {
+        conditionals: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (function() {
+            var b = a;
+            a ? a = b : (b++, a = b);
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a;
+        (function() {
+            var b = a;
+            a = (a ? 0 : b++, b);
+        })();
+        console.log(a);
+    }
+    expect_stdout: "NaN"
+}
+
+issue_5666_2: {
+    options = {
+        conditionals: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = "foo";
+        (function() {
+            var b = a;
+            a ? (b++, a = b) : a = b;
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "foo";
+        (function() {
+            var b = a;
+            a = (a ? b++ : 0, b);
+        })();
+        console.log(a);
+    }
+    expect_stdout: "NaN"
+}