diff --git a/Compatibility-issues.md b/Compatibility-issues.md
index eb6cdf2..ffd6f04 100644
--- a/Compatibility-issues.md
+++ b/Compatibility-issues.md
@@ -56,4 +56,14 @@ There is a temporary workaround for this issue: on your local copy of ArduinoJso
 
 Hopefully [the issue](https://github.com/PunchThrough/PunchThrough-BEAN-Arduino-Firmware/issues/23) will be fix in the core.
 
-See issue [#142](https://github.com/bblanchon/ArduinoJson/issues/142)
\ No newline at end of file
+See issue [#142](https://github.com/bblanchon/ArduinoJson/issues/142)
+
+## Known problems with Arduino IDE 1.6.6
+
+Arduino IDE 1.6.6 bug [#4075](https://github.com/arduino/Arduino/issues/4075) makes the build fail with the following error:
+
+```
+error: 'namespaceArduinoJson' does not name a type
+```
+
+You need to roll back to 1.6.5.
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