diff --git a/src/rpc/util.cpp b/src/rpc/util.cpp
--- a/src/rpc/util.cpp
+++ b/src/rpc/util.cpp
@@ -418,6 +418,10 @@
         std::string ret;
         const size_t pad = m_max_pad + 4;
         for (const auto &s : m_sections) {
+            // The left part of a section is assumed to be a single line,
+            // usually it is the name of the JSON struct or a brace like
+            // {, }, [, or ]
+            CHECK_NONFATAL(s.m_left.find('\n') == std::string::npos);
             if (s.m_right.empty()) {
                 ret += s.m_left;
                 ret += "\n";