--- a/media/webrtc/signaling/src/sipcc/cpr/common/cpr_string.c
+++ b/media/webrtc/signaling/src/sipcc/cpr/common/cpr_string.c
@@ -173,16 +173,24 @@ void flex_string_append(flex_string *fs,
 void flex_string_sprintf(flex_string *fs, const char *format, ...) {
   va_list ap;
   int vsnprintf_result;
 
   va_start(ap, format);
   vsnprintf_result = vsnprintf(fs->buffer + fs->string_length, fs->buffer_length - fs->string_length, format, ap);
   va_end(ap);
 
+  /* Special case just for Windows where vsnprintf is broken
+     and returns -1 if buffer too large unless you size it 0. */
+  if (vsnprintf_result < 0) {
+    va_start(ap, format);
+    vsnprintf_result = vsnprintf(NULL, 0, format, ap);
+    va_end(ap);
+  }
+
   if (fs->string_length + vsnprintf_result >= fs->buffer_length) {
     /* Buffer overflow, resize */
     flex_string_check_alloc(fs, fs->string_length + vsnprintf_result + 1);
 
     /* Try again with new buffer */
     va_start(ap, format);
     vsnprintf_result = vsnprintf(fs->buffer + fs->string_length, fs->buffer_length - fs->string_length, format, ap);
     MOZ_ASSERT(vsnprintf_result > 0 && vsnprintf_result < (fs->buffer_length - fs->string_length));