diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -1207,6 +1207,18 @@
                             // Disabled opcodes.
                             return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE);
                         }
+                        // (x1 x2 -- out)
+                        if (stack.size() < 2) {
+                            return set_error(
+                                serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+                        }
+                        valtype &vch1 = stacktop(-2);
+                        valtype &vch2 = stacktop(-1);
+                        if (vch1.size() + vch2.size() > MAX_SCRIPT_ELEMENT_SIZE) {
+                            return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
+                        }
+                        vch1.insert(vch1.end(), vch2.begin(), vch2.end());
+                        stack.pop_back();
                     } break;
 
                     case OP_SPLIT: {