diff --git a/Makefile b/Makefile
index 2a9a29fd0c..8e2f0aee4c 100644
--- a/Makefile
+++ b/Makefile
@@ -136,6 +136,7 @@ INC := -Iinclude -Iinclude/libc -Isrc -I$(BUILD_DIR) -I. -I$(EXTRACTED_DIR)
 
 # Check code syntax with host compiler
 CHECK_WARNINGS := -Wall -Wextra -Wno-format-security -Wno-unknown-pragmas -Wno-unused-parameter -Wno-unused-variable -Wno-missing-braces
+CHECK_WARNINGS += -Werror=implicit-function-declaration
 
 # The `cpp` command behaves differently on macOS (it behaves as if
 # `-traditional-cpp` was passed) so we use `gcc -E` instead.