diff --git a/src/stratum.cpp b/src/stratum.cpp
index 42e31a10d..16736e6e2 100644
--- a/src/stratum.cpp
+++ b/src/stratum.cpp
@@ -1334,7 +1334,16 @@ UniValue stratum_mining_submit(StratumClient& client, const UniValue& params)
         if (job_id_str.length() == 63) {
             fEWBFJobIDFixNeeded = true;
             for(const auto& hexDigit : hexDigits) {
-                ret = uint256(ParseHex(job_id_str + hexDigit));
+                // ParseHex() stops at the first non-hex character and returns a SHORT vector
+                // without signalling an error, and base_blob(const std::vector<unsigned char>&)
+                // asserts vch.size() == 32. Constructing without checking therefore lets any
+                // 63-character job_id containing a non-hex byte abort the daemon -- from an
+                // unauthenticated client, before any other validation. Skip bad candidates
+                // instead; if none of the 16 completions parse, ret stays null, misses
+                // work_templates below, and the handler returns false cleanly.
+                std::vector<unsigned char> vch = ParseHex(job_id_str + hexDigit);
+                if (vch.size() != 32) continue;
+                ret = uint256(vch);
                 if (work_templates.count(ret)) break;
             }
         }