diff --git a/Makefile b/Makefile
index 18ce8f2..9beb3c6 100644
--- a/Makefile
+++ b/Makefile
@@ -7,6 +7,7 @@ AR             = ar
 BUILD_TYPE     = Debug
 BUILD_DIR      = libwapp-build/$(PLATFORM)-$(BUILD_TYPE)
 INSTALL_PREFIX = dist
+CODEGEN_INPUT  = ""
 
 # Internal variables
 override CFLAGS              = -Wall -Wextra -Werror -pedantic -Isrc
@@ -54,6 +55,7 @@ help:
 	@echo "  BUILD_TYPE        Build type. Choose from \`Debug\`, \`RelWithDebInfo\` or \`Release\` (Default: Debug)."
 	@echo "  BUILD_DIR         Directory where build files will be written."
 	@echo "  INSTALL_PREFIX    Prefix where library and include files will be installed."
+	@echo "  CODEGEN_INPUT     Input file for code generation (See codegen_custom_data_example.json for an example)."
 	@echo
 	@echo "Available targets:"
 	@echo "  make              Build, install and test the full wapp library."
@@ -113,7 +115,7 @@ run-cc-test: build-cc-test
 	@rm $(TEST_CXX_OUT)
 
 codegen:
-	python3 -m codegen
+	python3 -m codegen -f $(CODEGEN_INPUT)
 
 install: build-lib
 	@mkdir -p $(LIB_INSTALL)
diff --git a/codegen/__main__.py b/codegen/__main__.py
index e2ea776..8a45e62 100644
--- a/codegen/__main__.py
+++ b/codegen/__main__.py
@@ -11,7 +11,7 @@ def main(types_file: Path | None):
     dbl_list_datatypes: Dict[CDataType, DblListData] = {}
     array_datatypes: Dict[CDataType, ArrayData] = {}
 
-    if types_file is not None:
+    if types_file is not None and types_file.is_file() and "json" in types_file.suffix.lower():
         with types_file.open("r") as infile:
             datatypes = json.load(infile)
             dbl_list_data = datatypes.get(DBL_LIST_DATA)