diff --git a/TESTS/mbed_hal/gpio/main.cpp b/TESTS/mbed_hal/gpio/main.cpp
new file mode 100644
index 00000000000..2281957b2f4
--- /dev/null
+++ b/TESTS/mbed_hal/gpio/main.cpp
@@ -0,0 +1,57 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2019 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "utest/utest.h"
+#include "unity/unity.h"
+#include "greentea-client/test_env.h"
+
+using namespace utest::v1;
+
+#include "PinNames.h"
+#include "gpio_api.h"
+
+static void gpio_nc_test()
+{
+    gpio_t nc_obj;
+    gpio_init(&nc_obj, NC);
+    TEST_ASSERT_FALSE(gpio_is_connected(&nc_obj));
+
+    gpio_t led_obj;
+    gpio_init(&led_obj, LED1);
+    if (LED1 == NC) {
+        TEST_ASSERT_FALSE(gpio_is_connected(&led_obj));
+    } else {
+        TEST_ASSERT_TRUE(gpio_is_connected(&led_obj));
+    }
+}
+
+Case cases[] = {
+    Case("gpio NC test", gpio_nc_test)
+};
+
+utest::v1::status_t greentea_test_setup(const size_t number_of_cases)
+{
+    GREENTEA_SETUP(20, "default_auto");
+    return greentea_test_setup_handler(number_of_cases);
+}
+
+Specification specification(greentea_test_setup, cases, greentea_test_teardown_handler);
+
+int main()
+{
+    Harness::run(specification);
+}
diff --git a/hal/gpio_api.h b/hal/gpio_api.h
index 51156acd23f..503e682a6f5 100644
--- a/hal/gpio_api.h
+++ b/hal/gpio_api.h
@@ -29,6 +29,16 @@ extern "C" {
 
 /**
  * \defgroup hal_gpio GPIO HAL functions
+ *
+ * # Defined behavior
+ * * ::gpio_init and other init functions can be called with NC or a valid PinName for the target - Verified by ::gpio_nc_test
+ * * ::gpio_is_connected can be used to test whether a gpio_t object was initialized with NC - Verified by ::gpio_nc_test
+ *
+ * # Undefined behavior
+ * * Calling any ::gpio_mode, ::gpio_dir, ::gpio_write or ::gpio_read on a gpio_t object that was initialized
+ *   with NC.
+ * * Calling ::gpio_set with NC.
+ *
  * @{
  */
 
@@ -38,43 +48,45 @@ extern "C" {
  * @return The GPIO port mask for this pin
  **/
 uint32_t gpio_set(PinName pin);
-/* Checks if gpio object is connected (pin was not initialized with NC)
- * @param pin The pin to be set as GPIO
- * @return 0 if port is initialized with NC
+
+/** Checks if gpio object is connected (pin was not initialized with NC)
+ * @param obj The GPIO object
+ * @return 0 if object was initialized with NC
+ * @return non-zero if object was initialized with a valid PinName
  **/
 int gpio_is_connected(const gpio_t *obj);
 
 /** Initialize the GPIO pin
  *
  * @param obj The GPIO object to initialize
- * @param pin The GPIO pin to initialize
+ * @param pin The GPIO pin to initialize (may be NC)
  */
 void gpio_init(gpio_t *obj, PinName pin);
 
 /** Set the input pin mode
  *
- * @param obj  The GPIO object
+ * @param obj  The GPIO object (must be connected)
  * @param mode The pin mode to be set
  */
 void gpio_mode(gpio_t *obj, PinMode mode);
 
 /** Set the pin direction
  *
- * @param obj       The GPIO object
+ * @param obj       The GPIO object (must be connected)
  * @param direction The pin direction to be set
  */
 void gpio_dir(gpio_t *obj, PinDirection direction);
 
 /** Set the output value
  *
- * @param obj   The GPIO object
+ * @param obj   The GPIO object (must be connected)
  * @param value The value to be set
  */
 void gpio_write(gpio_t *obj, int value);
 
 /** Read the input value
  *
- * @param obj The GPIO object
+ * @param obj The GPIO object (must be connected)
  * @return An integer value 1 or 0
  */
 int gpio_read(gpio_t *obj);
@@ -85,14 +97,14 @@ int gpio_read(gpio_t *obj);
 /** Init the input pin and set mode to PullDefault
  *
  * @param gpio The GPIO object
- * @param pin  The pin name
+ * @param pin  The pin name (may be NC)
  */
 void gpio_init_in(gpio_t *gpio, PinName pin);
 
 /** Init the input pin and set the mode
  *
  * @param gpio  The GPIO object
- * @param pin   The pin name
+ * @param pin   The pin name (may be NC)
  * @param mode  The pin mode to be set
  */
 void gpio_init_in_ex(gpio_t *gpio, PinName pin, PinMode mode);
@@ -100,7 +112,7 @@ void gpio_init_in_ex(gpio_t *gpio, PinName pin, PinMode mode);
 /** Init the output pin as an output, with predefined output value 0
  *
  * @param gpio The GPIO object
- * @param pin  The pin name
+ * @param pin  The pin name (may be NC)
  * @return     An integer value 1 or 0
  */
 void gpio_init_out(gpio_t *gpio, PinName pin);
@@ -108,7 +120,7 @@ void gpio_init_out(gpio_t *gpio, PinName pin);
 /** Init the pin as an output and set the output value
  *
  * @param gpio  The GPIO object
- * @param pin   The pin name
+ * @param pin   The pin name (may be NC)
  * @param value The value to be set
  */
 void gpio_init_out_ex(gpio_t *gpio, PinName pin, int value);
@@ -116,7 +128,7 @@ void gpio_init_out_ex(gpio_t *gpio, PinName pin, int value);
 /** Init the pin to be in/out
  *
  * @param gpio      The GPIO object
- * @param pin       The pin name
+ * @param pin       The pin name (may be NC)
  * @param direction The pin direction to be set
  * @param mode      The pin mode to be set
  * @param value     The value to be set for an output pin
diff --git a/targets/TARGET_ARM_FM/TARGET_FVP_MPS2/gpio_api.c b/targets/TARGET_ARM_FM/TARGET_FVP_MPS2/gpio_api.c
index 2da2b84e53e..5cfdd573c70 100644
--- a/targets/TARGET_ARM_FM/TARGET_FVP_MPS2/gpio_api.c
+++ b/targets/TARGET_ARM_FM/TARGET_FVP_MPS2/gpio_api.c
@@ -27,11 +27,11 @@ uint32_t gpio_set(PinName pin)
 // with the object created for the pin
 void gpio_init(gpio_t *obj, PinName pin)
 {
+    obj->pin = pin;
     if (pin == NC) {
         return;
     } else {
         int pin_value = 0;
-        obj->pin = pin;
         if (pin <= 15) {
             pin_value = pin;
         } else if (pin >= 16 && pin <= 31) {
diff --git a/targets/TARGET_ARM_SSG/TARGET_CM3DS_MPS2/gpio_api.c b/targets/TARGET_ARM_SSG/TARGET_CM3DS_MPS2/gpio_api.c
index 46a95c4abfd..2458569ae79 100644
--- a/targets/TARGET_ARM_SSG/TARGET_CM3DS_MPS2/gpio_api.c
+++ b/targets/TARGET_ARM_SSG/TARGET_CM3DS_MPS2/gpio_api.c
@@ -101,6 +101,13 @@ void gpio_init(gpio_t *obj, PinName pin)
 {
     struct arm_gpio_dev_t *gpio_dev;
 
+    if (pin == NC) {
+        obj->pin_number = NC;
+        obj->gpio_dev = NULL;
+        obj->mps2_io_dev = NULL;
+        return;
+    }
+
     if (pin >= EXP0 && pin <= EXP51) {
         /* GPIO pins */
         switch (GPIO_DEV_NUMBER(pin)) {
diff --git a/targets/TARGET_ARM_SSG/TARGET_IOTSS/gpio_api.c b/targets/TARGET_ARM_SSG/TARGET_IOTSS/gpio_api.c
index b80fcbacd17..2a87bb2dda6 100644
--- a/targets/TARGET_ARM_SSG/TARGET_IOTSS/gpio_api.c
+++ b/targets/TARGET_ARM_SSG/TARGET_IOTSS/gpio_api.c
@@ -25,10 +25,10 @@ uint32_t gpio_set(PinName pin) {
 // this links the board control bits for each pin 
 // with the object created for the pin
 void gpio_init(gpio_t *obj, PinName pin) {
+        obj->pin = pin;
         if(pin == NC){ return;}
         else {
         int pin_value = 0;
-        obj->pin = pin;
         if(pin <=15){
             pin_value = pin;
         }else if (pin >= 16 && pin <= 31){
diff --git a/targets/TARGET_Atmel/TARGET_SAM_CortexM0P/gpio_api.c b/targets/TARGET_Atmel/TARGET_SAM_CortexM0P/gpio_api.c
index 2706e3cfb36..66bf6ca4cc2 100644
--- a/targets/TARGET_Atmel/TARGET_SAM_CortexM0P/gpio_api.c
+++ b/targets/TARGET_Atmel/TARGET_SAM_CortexM0P/gpio_api.c
@@ -28,7 +28,6 @@ uint32_t gpio_set(PinName pin)
 
 void gpio_init(gpio_t *obj, PinName pin)
 {
-    MBED_ASSERT(pin != (PinName)NC);
     struct port_config pin_conf;
     PortGroup *const port_base = (PortGroup*)port_get_group_from_gpio_pin(pin);
 
diff --git a/targets/TARGET_Atmel/TARGET_SAM_CortexM4/gpio_api.c b/targets/TARGET_Atmel/TARGET_SAM_CortexM4/gpio_api.c
index bb1af212582..b6613e4d1c9 100644
--- a/targets/TARGET_Atmel/TARGET_SAM_CortexM4/gpio_api.c
+++ b/targets/TARGET_Atmel/TARGET_SAM_CortexM4/gpio_api.c
@@ -30,13 +30,15 @@ uint32_t gpio_set(PinName pin)
 
 void gpio_init(gpio_t *obj, PinName pin)
 {
-    MBED_ASSERT(pin != (PinName)NC);
     if (g_sys_init == 0) {
         sysclk_init();
         system_board_init();
         g_sys_init = 1;
     }
     obj->pin = pin;
+    if (pin == NC) {
+        return;
+    }
 
     ioport_set_pin_dir(pin, IOPORT_DIR_INPUT);
     ioport_set_pin_mode(pin, IOPORT_MODE_PULLUP);
diff --git a/targets/TARGET_Realtek/TARGET_AMEBA/TARGET_MCU_RTL8195A/gpio_api.c b/targets/TARGET_Realtek/TARGET_AMEBA/TARGET_MCU_RTL8195A/gpio_api.c
index cf81673e18e..56285489a2e 100644
--- a/targets/TARGET_Realtek/TARGET_AMEBA/TARGET_MCU_RTL8195A/gpio_api.c
+++ b/targets/TARGET_Realtek/TARGET_AMEBA/TARGET_MCU_RTL8195A/gpio_api.c
@@ -83,10 +83,11 @@ void gpio_init(gpio_t *obj, PinName pin)
 {
     uint32_t pin_name;
 
+    obj->pin = pin;
+
     if (pin == (PinName)NC)
         return;
 
-    obj->pin = pin;
     obj->mode = PullNone;
     obj->direction = PIN_INPUT;
     pin_name = gpio_set(pin); // get the IP pin name
diff --git a/targets/TARGET_TT/TARGET_TT_M3HQ/gpio_api.c b/targets/TARGET_TT/TARGET_TT_M3HQ/gpio_api.c
index ae96d4ae601..a800f57e2ed 100644
--- a/targets/TARGET_TT/TARGET_TT_M3HQ/gpio_api.c
+++ b/targets/TARGET_TT/TARGET_TT_M3HQ/gpio_api.c
@@ -36,9 +36,11 @@ void gpio_init(gpio_t *obj, PinName pin)
 {
     // Store above pin mask, pin name into GPIO object
     obj->pin = pin;
-    obj->mask = gpio_set(pin);
-    obj->port = (PortName) (pin >> 3);
-    TSB_CG->FSYSENA |= (1<<(obj->port));
+    if (pin != NC) {
+        obj->mask = gpio_set(pin);
+        obj->port = (PortName) (pin >> 3);
+        TSB_CG->FSYSENA |= (1<<(obj->port));
+    }
 }
 
 void gpio_mode(gpio_t *obj, PinMode mode)
diff --git a/targets/TARGET_TT/TARGET_TT_M4G9/gpio_api.c b/targets/TARGET_TT/TARGET_TT_M4G9/gpio_api.c
index 2e7603750b4..c30f3f1f450 100644
--- a/targets/TARGET_TT/TARGET_TT_M4G9/gpio_api.c
+++ b/targets/TARGET_TT/TARGET_TT_M4G9/gpio_api.c
@@ -61,11 +61,13 @@ void gpio_init(gpio_t *obj, PinName pin)
 {
     // Store above pin mask, pin name into GPIO object
     obj->pin      = pin;
-    obj->pin_num  = PIN_POS(pin);
-    obj->mask     = gpio_set(pin);
-    obj->port     = (PortName) PIN_PORT(pin);
-    //Enable clock for particular port
-    TSB_CG->FSYSMENB |= (1 << ((obj->port) + 2));
+    if (pin != NC) {
+        obj->pin_num  = PIN_POS(pin);
+        obj->mask     = gpio_set(pin);
+        obj->port     = (PortName) PIN_PORT(pin);
+        //Enable clock for particular port
+        TSB_CG->FSYSMENB |= (1 << ((obj->port) + 2));
+    }
 }
 
 void gpio_mode(gpio_t *obj, PinMode mode)
diff --git a/targets/TARGET_WIZNET/TARGET_W7500x/gpio_api.c b/targets/TARGET_WIZNET/TARGET_W7500x/gpio_api.c
index d40f2f01c71..9747c01b2d9 100644
--- a/targets/TARGET_WIZNET/TARGET_W7500x/gpio_api.c
+++ b/targets/TARGET_WIZNET/TARGET_W7500x/gpio_api.c
@@ -45,13 +45,13 @@ uint32_t gpio_set(PinName pin)
 
 void gpio_init(gpio_t *obj, PinName pin)
 {
+    obj->pin = pin;
     if (pin == (PinName)NC) {
         return;
     }
     
     obj->port_num = WIZ_PORT(pin);
     obj->pin_index    = WIZ_PIN_INDEX(pin);
-    obj->pin = pin;
     GPIO_TypeDef *gpio = (GPIO_TypeDef *)Get_GPIO_BaseAddress(obj->port_num);
     obj->reg_data_in    = &gpio->DATA;
 }
diff --git a/targets/TARGET_ublox/TARGET_HI2110/gpio_api.c b/targets/TARGET_ublox/TARGET_HI2110/gpio_api.c
index 7024631750a..0406d2a0262 100644
--- a/targets/TARGET_ublox/TARGET_HI2110/gpio_api.c
+++ b/targets/TARGET_ublox/TARGET_HI2110/gpio_api.c
@@ -36,13 +36,13 @@
 
 void gpio_init(gpio_t *obj, PinName pin)
 {
+    obj->pin = pin;
     if (pin == (PinName)NC) {
         return;
     }
     
     MBED_ASSERT (pin <  NUM_PINS);
 
-    obj->pin = pin;
     obj->mask = (1ul << pin);
 
     obj->reg_set = &GPIO_OUT_BITSET;