diff --git a/libraries/mbed/vendor/NXP/cmsis/LPC11U24/GCC_CS/sys.cpp b/libraries/mbed/vendor/NXP/cmsis/LPC11U24/GCC_CS/sys.cpp
index e4ec5b523a..6fc5736daf 100644
--- a/libraries/mbed/vendor/NXP/cmsis/LPC11U24/GCC_CS/sys.cpp
+++ b/libraries/mbed/vendor/NXP/cmsis/LPC11U24/GCC_CS/sys.cpp
@@ -76,8 +76,4 @@ void *_sbrk(unsigned int incr) {
     return mem;
 }
 
-int _isatty(int file) {
-    return (file < 3) ? 1 : 0;
-}
-
 }
diff --git a/libraries/mbed/vendor/NXP/cmsis/LPC1768/GCC_CS/sys.cpp b/libraries/mbed/vendor/NXP/cmsis/LPC1768/GCC_CS/sys.cpp
index 8260e20335..9eb0108e06 100644
--- a/libraries/mbed/vendor/NXP/cmsis/LPC1768/GCC_CS/sys.cpp
+++ b/libraries/mbed/vendor/NXP/cmsis/LPC1768/GCC_CS/sys.cpp
@@ -77,8 +77,4 @@ void *_sbrk(unsigned int incr) {
     return mem;
 }
 
-int _isatty(int file) {
-    return (file < 3) ? 1 : 0;
-}
-
 }