diff --git a/cmake/common/px4_base.cmake b/cmake/common/px4_base.cmake
index 086f77070f..5c9b8aefdf 100644
--- a/cmake/common/px4_base.cmake
+++ b/cmake/common/px4_base.cmake
@@ -421,7 +421,8 @@ function(px4_add_upload)
 			)
 	elseif(${CMAKE_HOST_SYSTEM_NAME} STREQUAL "Windows")
 		foreach(port RANGE 32 0)
-			list(APPEND "COM${port}")
+			list(APPEND serial_ports
+				"COM${port}")
 		endforeach()
 	endif()
 	px4_join(OUT serial_ports LIST "${serial_ports}" GLUE ",")