diff --git a/pio-tools/add_c_flags.py b/pio-tools/add_c_flags.py
index c93059b36..0c39e777f 100644
--- a/pio-tools/add_c_flags.py
+++ b/pio-tools/add_c_flags.py
@@ -1,4 +1,4 @@
 Import("env")
 
 # General options that are passed to the C compiler (C only; not C++).
-env.Append(CFLAGS=["-Wno-discarded-qualifiers"])
+env.Append(CFLAGS=["-Wno-discarded-qualifiers", "-Wno-implicit-function-declaration"])