diff --git a/src/strings.cpp b/src/strings.cpp
index f6df675b28..12bf512a8b 100644
--- a/src/strings.cpp
+++ b/src/strings.cpp
@@ -1113,6 +1113,11 @@ static void FormatString(StringBuilder &builder, std::string_view str_arg, Strin
 			const uint case_index = str_stack.top().case_index;
 			char32_t b = consumer.ReadUtf8();
 			assert(b != 0);
+			if (b == 0) {
+				/* A NUL character should never be encountered, but for non-debug builds handle it gracefully. */
+				builder += "(unexpected NUL)";
+				continue;
+			}
 
 			if (SCC_NEWGRF_FIRST <= b && b <= SCC_NEWGRF_LAST) {
 				/* We need to pass some stuff as it might be modified. */