--- a/parser/html/nsHtml5TreeBuilder.cpp
+++ b/parser/html/nsHtml5TreeBuilder.cpp
@@ -394,16 +394,17 @@ nsHtml5TreeBuilder::characters(const cha
                   accumulateCharacters(buf, start, i - start);
                   start = i;
                 }
                 flushCharacters();
                 reconstructTheActiveFormattingElements();
                 continue;
               }
             }
+          MOZ_FALLTHROUGH_ASSERT("unreachable");
           }
           default: {
             switch (mode) {
               case INITIAL: {
                 documentModeInternal(QUIRKS_MODE, nullptr, nullptr, false);
                 mode = BEFORE_HTML;
                 i--;
                 continue;