diff --git a/Compatibility-issues.md b/Compatibility-issues.md
index 34a4e4f..e4af8d4 100644
--- a/Compatibility-issues.md
+++ b/Compatibility-issues.md
@@ -136,4 +136,20 @@ See issue [#284](https://github.com/bblanchon/ArduinoJson/issues/284)
 
 For some reason, it's not possible to use ArduinoJson with CodeBender.
 
-See discussion here: http://feedback.codebender.cc/forums/280538-library-requests/suggestions/7238254-arduinojson-support
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+See discussion here: http://feedback.codebender.cc/forums/280538-library-requests/suggestions/7238254-arduinojson-support
+
+## Arduino Zero and mkr1000 (all boards based on SAMD21)
+
+There is a bug in [Arduino SAMD core](https://github.com/arduino/ArduinoCore-samd) that make `prettyPrintTo()` fail.
+
+It's been fixed on May 27th by [arduino/ArduinoCore-samd@8ef9bb3](https://github.com/arduino/ArduinoCore-samd/commit/8ef9bb3d7bd026b57a8568030d8992644c9b8de8) and [arduino/ArduinoCore-samd@8d0c167](https://github.com/arduino/ArduinoCore-samd/commit/8d0c1674628df1c2c7592f4fc17467c694f5a1be).
+Unfortunately, version 1.6.6 only includes modifications up to May 19th, so the fix is not available through the Arduino Boards Manager.
+
+I recommend to apply the modification manually or wait until the next release of the Arduino SAMD Core.
+The files to change are [`USBCore.cpp`](https://github.com/arduino/ArduinoCore-samd/commit/8ef9bb3d7bd026b57a8568030d8992644c9b8de8) and [`CDC.cpp`](https://github.com/arduino/ArduinoCore-samd/commit/8d0c1674628df1c2c7592f4fc17467c694f5a1be). 
+
+They are located here on Windows:
+
+```
+%LOCALAPPDATA%\Arduino15\packages\arduino\hardware\samd\1.6.6\cores\arduino\USB
+```
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