diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -98,6 +98,34 @@
     return false;
 }
 
+namespace {
+/**
+ * A data type to abstract out the condition stack during script execution.
+ *
+ * Conceptually it acts like a vector of booleans, one for each level of nested
+ * IF/THEN/ELSE, indicating whether we're in the active or inactive branch of
+ * each.
+ *
+ * The elements on the stack cannot be observed individually; we only need to
+ * expose whether the stack is empty and whether or not any false values are
+ * present at all. To implement OP_ELSE, a toggle_top modifier is added, which
+ * flips the last value without returning it.
+ */
+class ConditionStack {
+private:
+    std::vector<bool> m_flags;
+
+public:
+    bool empty() { return m_flags.empty(); }
+    bool all_true() {
+        return !std::count(m_flags.begin(), m_flags.end(), false);
+    }
+    void push_back(bool f) { m_flags.push_back(f); }
+    void pop_back() { m_flags.pop_back(); }
+    void toggle_top() { m_flags.back() = !m_flags.back(); }
+};
+} // namespace
+
 bool EvalScript(std::vector<valtype> &stack, const CScript &script,
                 uint32_t flags, const BaseSignatureChecker &checker,
                 ScriptExecutionMetrics &metrics, ScriptError *serror) {
@@ -111,7 +139,7 @@
     CScript::const_iterator pbegincodehash = script.begin();
     opcodetype opcode;
     valtype vchPushValue;
-    std::vector<bool> vfExec;
+    ConditionStack vfExec;
     std::vector<valtype> altstack;
     set_error(serror, ScriptError::UNKNOWN);
     if (script.size() > MAX_SCRIPT_SIZE) {
@@ -122,7 +150,7 @@
 
     try {
         while (pc < pend) {
-            bool fExec = !count(vfExec.begin(), vfExec.end(), false);
+            bool fExec = vfExec.all_true();
 
             //
             // Read instruction
@@ -324,7 +352,7 @@
                             return set_error(
                                 serror, ScriptError::UNBALANCED_CONDITIONAL);
                         }
-                        vfExec.back() = !vfExec.back();
+                        vfExec.toggle_top();
                     } break;
 
                     case OP_ENDIF: {