diff --git a/buildroot/share/PlatformIO/scripts/preprocessor.py b/buildroot/share/PlatformIO/scripts/preprocessor.py
index efc3a6261e..3cd279fa57 100644
--- a/buildroot/share/PlatformIO/scripts/preprocessor.py
+++ b/buildroot/share/PlatformIO/scripts/preprocessor.py
@@ -37,15 +37,20 @@ def run_preprocessor(env, fn=None):
         else:
             cmd += ['-D' + s]
 
-    cmd += ['-D__MARLIN_DEPS__ -w -dM -E -x c++']
-    depcmd = cmd + [ filename ]
-    cmd = ' '.join(depcmd)
+    cmd += ['-D__MARLIN_DEPS__ -w -dM -E -x c++', filename]
+
+    cmd = ' '.join(cmd)
     blab(cmd)
+
     try:
-        define_list = subprocess.check_output(cmd, shell=True).splitlines()
+        define_list_text = subprocess.check_output(cmd, shell=True)
     except:
-        define_list = {}
+        raise RuntimeError(f"Command `{cmd}` failed during build pre-processing.")
+
+    define_list = define_list_text.splitlines() if define_list_text else []
+
     preprocessor_cache[filename] = define_list
+
     return define_list