--- a/js/src/vm/CodeCoverage.cpp
+++ b/js/src/vm/CodeCoverage.cpp
@@ -566,17 +566,17 @@ LCovRuntime::fillWithFilename(char *name
         return false;
 
     int64_t timestamp = static_cast<double>(PRMJ_Now()) / PRMJ_USEC_PER_SEC;
     static mozilla::Atomic<size_t> globalRuntimeId(0);
     size_t rid = globalRuntimeId++;
 
     int len = snprintf(name, length, "%s/%" PRId64 "-%" PRIu32 "-%" PRIuSIZE ".info",
                        outDir, timestamp, pid_, rid);
-    if (length != size_t(len)) {
+    if (len < 0 || size_t(len) >= length) {
         fprintf(stderr, "Warning: LCovRuntime::init: Cannot serialize file name.");
         return false;
     }
 
     return true;
 }
 
 void