diff --git a/features/frameworks/greentea-client/greentea-client/test_env_c.h b/features/frameworks/greentea-client/greentea-client/test_env_c.h
index 67beceb6c7..352b749b66 100644
--- a/features/frameworks/greentea-client/greentea-client/test_env_c.h
+++ b/features/frameworks/greentea-client/greentea-client/test_env_c.h
@@ -28,7 +28,7 @@ void GREENTEA_SETUP(const int timeout, const char * host_test);
 void greentea_send_kv(const char * key, const char * val);
 int greentea_parse_kv(char * key, char * val,
                         const int key_len, const int val_len);
-char greentea_getc();
+int greentea_getc();
 
 #endif /* TEST_ENV_C_H */
 
diff --git a/features/frameworks/greentea-client/source/greentea_test_env.cpp b/features/frameworks/greentea-client/source/greentea_test_env.cpp
index f2c4298474..8b4ee14193 100644
--- a/features/frameworks/greentea-client/source/greentea_test_env.cpp
+++ b/features/frameworks/greentea-client/source/greentea_test_env.cpp
@@ -554,7 +554,7 @@ enum Token {
  * \return Next character from the stream or EOF if stream has ended.
  *
  */
-extern "C" char greentea_getc() {
+extern "C" int greentea_getc() {
     return greentea_serial->getc();
 }
 
@@ -683,7 +683,7 @@ static int isstring(int c) {
  *
  */
 static int gettok(char *out_str, const int str_size) {
-    static char LastChar = '!';
+    static int LastChar = '!';
     static int str_idx = 0;
 
     // whitespace ::=