--- a/xpcom/string/nsTextFormatter.cpp
+++ b/xpcom/string/nsTextFormatter.cpp
@@ -449,16 +449,20 @@ nsTextFormatter::cvt_S(SprintfStateStr* 
 ** Convert a string into its printable form.  |aWidth| is the output
 ** width. |aPrec| is the maximum number of characters of |aStr| to output,
 ** where -1 means until NUL.
 */
 int
 nsTextFormatter::cvt_s(nsTextFormatter::SprintfStateStr* aState, const char* aStr, int aWidth,
                        int aPrec, int aFlags)
 {
+  // Be sure to handle null the same way as %S.
+  if (aStr == nullptr) {
+    return cvt_S(aState, nullptr, aWidth, aPrec, aFlags);
+  }
   NS_ConvertUTF8toUTF16 utf16Val(aStr);
   return cvt_S(aState, utf16Val.get(), aWidth, aPrec, aFlags);
 }
 
 /*
 ** The workhorse sprintf code.
 */
 int