--- a/mozglue/misc/Printf.cpp
+++ b/mozglue/misc/Printf.cpp
@@ -271,19 +271,20 @@ mozilla::PrintfTarget::cvt_f(double d, c
     {
         const char* p = fin;
         while (*p) {
             MOZ_ASSERT(*p != 'L');
             p++;
         }
     }
 #endif
-    SprintfLiteral(fout, fin, d);
+    size_t len = SprintfLiteral(fout, fin, d);
+    MOZ_ASSERT(len <= sizeof(fout));
 
-    return emit(fout, strlen(fout));
+    return emit(fout, len);
 }
 
 /*
  * Convert a string into its printable form.  "width" is the output
  * width. "prec" is the maximum number of characters of "s" to output,
  * where -1 means until NUL.
  */
 bool