diff --git a/HISTORY.rst b/HISTORY.rst
index 44c43bb7..7280d349 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -15,6 +15,8 @@ PlatformIO 3.0
 * Support in-line comments for multi-line value (``lib_deps``, ``build_flags``, etc) in `“platformio.ini” (Project Configuration File) <https://docs.platformio.org/page/projectconf.html>`__
 * Do not re-create ".gitignore" and ".travis.yml" files if they were removed
   from a project
+* Fixed an issue when dynamic build flags were not handled correctly
+  (`issue #1799 <https://github.com/platformio/platformio-core/issues/1799>`_)
 
 3.6.0 (2018-08-06)
 ~~~~~~~~~~~~~~~~~~
diff --git a/platformio/builder/tools/platformio.py b/platformio/builder/tools/platformio.py
index 9346f235..27726756 100644
--- a/platformio/builder/tools/platformio.py
+++ b/platformio/builder/tools/platformio.py
@@ -142,9 +142,14 @@ def BuildProgram(env):
 
 
 def ParseFlagsExtended(env, flags):
-    if isinstance(flags, list):
-        flags = " ".join(flags)
-    result = env.ParseFlags(str(flags))
+    if not isinstance(flags, list):
+        flags = [flags]
+    result = {}
+    for raw in flags:
+        for key, value in env.ParseFlags(str(raw)).items():
+            if key not in result:
+                result[key] = []
+            result[key].extend(value)
 
     cppdefines = []
     for item in result['CPPDEFINES']: