diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1253,6 +1253,7 @@
         if (pvChecks) {
             pvChecks->push_back(std::move(check));
         } else if (!check()) {
+            ScriptError scriptError = check.GetScriptError();
             // Compute flags without the optional standardness flags.
             // This differs from MANDATORY_SCRIPT_VERIFY_FLAGS as it contains
             // additional upgrade flags (see AcceptToMemoryPoolWorker variable
@@ -1270,19 +1271,15 @@
                     return state.Invalid(
                         false, REJECT_NONSTANDARD,
                         strprintf("non-mandatory-script-verify-flag (%s)",
-                                  ScriptErrorString(check.GetScriptError())));
+                                  ScriptErrorString(scriptError)));
                 }
+                // update the error message to reflect the mandatory violation.
+                scriptError = check2.GetScriptError();
             }
 
             // Before banning, we need to check whether the transaction would
             // be valid on the other side of the upgrade, so as to avoid
             // splitting the network between upgraded and non-upgraded nodes.
-            // Note that this will create strange error messages like
-            // "upgrade-conditional-script-failure (Dummy CHECKMULTISIG argument
-            // must be zero)" -- the tx was initially refused entry due to
-            // NULLDUMMY, a standardness flag, but it is outright invalid before
-            // the upgrade as it contains schnorr signatures; it would however
-            // be valid after the upgrade.
             CScriptCheck check3(scriptPubKey, amount, tx, i,
                                 mandatoryFlags ^ SCRIPT_ENABLE_SCHNORR_MULTISIG,
                                 sigCacheStore, txdata);
@@ -1290,7 +1287,7 @@
                 return state.Invalid(
                     false, REJECT_INVALID,
                     strprintf("upgrade-conditional-script-failure (%s)",
-                              ScriptErrorString(check.GetScriptError())));
+                              ScriptErrorString(scriptError)));
             }
 
             // Failures of other flags indicate a transaction that is invalid in
@@ -1302,7 +1299,7 @@
             return state.DoS(
                 100, false, REJECT_INVALID,
                 strprintf("mandatory-script-verify-flag-failed (%s)",
-                          ScriptErrorString(check.GetScriptError())));
+                          ScriptErrorString(scriptError)));
         }
     }
 
diff --git a/test/functional/abc-schnorrmultisig-activation.py b/test/functional/abc-schnorrmultisig-activation.py
--- a/test/functional/abc-schnorrmultisig-activation.py
+++ b/test/functional/abc-schnorrmultisig-activation.py
@@ -59,7 +59,7 @@
 
 # Before the upgrade, Schnorr checkmultisig is rejected but forgiven if it would have been valid after the upgrade.
 PREUPGRADE_SCHNORR_MULTISIG_ERROR = dict(reject_code=16,
-                                         reject_reason=b'upgrade-conditional-script-failure (Dummy CHECKMULTISIG argument must be zero)')
+                                         reject_reason=b'upgrade-conditional-script-failure (Signature cannot be 65 bytes in CHECKMULTISIG)')
 
 # Before the upgrade, ECDSA checkmultisig with non-null dummy are rejected with a non-mandatory error.
 PREUPGRADE_ECDSA_NULLDUMMY_ERROR = dict(reject_code=64,