diff --git a/HISTORY.rst b/HISTORY.rst
index 4127fdca..3d31d53e 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -11,6 +11,8 @@ PlatformIO Core 5
 5.2.1 (2021-??-??)
 ~~~~~~~~~~~~~~~~~~
 
+- Fixed a "KeyError: Invalid board option 'build.cpu'" when using a precompiled library with a board that does not have a CPU field in the manifest (`issue #4056 <https://github.com/platformio/platformio-core/issues/4056>`_)
+
 5.2.0 (2021-09-13)
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/platformio/builder/tools/piolib.py b/platformio/builder/tools/piolib.py
index 186485d2..dfe781e0 100644
--- a/platformio/builder/tools/piolib.py
+++ b/platformio/builder/tools/piolib.py
@@ -567,7 +567,7 @@ class ArduinoLibBuilder(LibBuilderBase):
             # add to LDPATH {build.mcu} folder
             board_config = self.env.BoardConfig()
             self.env.PrependUnique(
-                LIBPATH=os.path.join(self.src_dir, board_config.get("build.cpu"))
+                LIBPATH=os.path.join(self.src_dir, board_config.get("build.cpu", ""))
             )
         ldflags = [flag for flag in ldflags if flag]  # remove empty
         return " ".join(ldflags) if ldflags else None