diff --git a/tools/config/__init__.py b/tools/config/__init__.py
index e6d888fda3..ee491c99df 100644
--- a/tools/config/__init__.py
+++ b/tools/config/__init__.py
@@ -372,6 +372,20 @@ class Config(object):
         "LOWPAN_BORDER_ROUTER", "LOWPAN_HOST", "LOWPAN_ROUTER", "NANOSTACK_FULL", "THREAD_BORDER_ROUTER", "THREAD_END_DEVICE", "THREAD_ROUTER", "ETHERNET_HOST"
         ]
 
+    @classmethod
+    def find_app_config(cls, top_level_dirs):
+        app_config_location = None
+        for directory in top_level_dirs:
+            full_path = os.path.join(directory, cls.__mbed_app_config_name)
+            if os.path.isfile(full_path):
+                if app_config_location is not None:
+                    raise ConfigException("Duplicate '%s' file in '%s' and '%s'"
+                                            % (cls.__mbed_app_config_name,
+                                               cls.app_config_location, full_path))
+                else:
+                    app_config_location = full_path
+        return app_config_location
+
     def __init__(self, tgt, top_level_dirs=None, app_config=None):
         """Construct a mbed configuration
 
@@ -391,16 +405,8 @@ class Config(object):
         """
         config_errors = []
         self.app_config_location = app_config
-        if self.app_config_location is None:
-            for directory in top_level_dirs or []:
-                full_path = os.path.join(directory, self.__mbed_app_config_name)
-                if os.path.isfile(full_path):
-                    if self.app_config_location is not None:
-                        raise ConfigException("Duplicate '%s' file in '%s' and '%s'"
-                                              % (self.__mbed_app_config_name,
-                                                 self.app_config_location, full_path))
-                    else:
-                        self.app_config_location = full_path
+        if self.app_config_location is None and top_level_dirs:
+            self.app_config_location = self.find_app_config(top_level_dirs)
         try:
             self.app_config_data = json_file_to_dict(self.app_config_location) \
                                    if self.app_config_location else {}
diff --git a/tools/test_configs/__init__.py b/tools/test_configs/__init__.py
index 4b30632ca2..4f884465e7 100644
--- a/tools/test_configs/__init__.py
+++ b/tools/test_configs/__init__.py
@@ -2,6 +2,7 @@ from os.path import dirname, abspath, join, exists
 
 from tools.utils import json_file_to_dict
 from tools.targets import TARGET_MAP
+from tools.config import Config
 
 CONFIG_DIR = dirname(abspath(__file__))
 CONFIG_MAP = json_file_to_dict(join(CONFIG_DIR, "config_paths.json"))
@@ -34,8 +35,8 @@ def get_default_config(source_dir, target_name):
         if config_name == "NONE":
             return None
         return join(CONFIG_DIR, CONFIG_MAP[config_name])
-    elif any(exists(join(dir or ".", "mbed_app.json")) for dir in source_dir):
-        config = None
+    elif Config.find_app_config(source_dir):
+        return None
     elif (target_name in TARGET_MAP and 'LWIP' in TARGET_MAP[target_name].features):
         return join(CONFIG_DIR, CONFIG_MAP["ETHERNET"])
     else: